Always keep a change output, so every tx stays fee-bumpable (B-62)

The max-amount checkbox sends amount_sats == the whole confirmed balance, so
change came out at 0, the change output was dropped, and the transaction had a
single output. bump_fee has nothing to shrink there: it raised RbfError every
30s until the reconciler abandoned the row six hours later. The RBF
single-change-output limitation was a documented gap, but the UI made it the
*default* withdrawal path.

The extra-input fallback would not have helped this case: a transaction moving
the entire balance already spends every UTXO the sender has. So the fix is at
build time — build_signed_transaction never produces a change output below
DUST_LIMIT_SATS, and never folds it into the fee either:

- withdrawals pass reduce_amount_to_keep_change=True and move a dust limit less.
  The fee already comes out of the withdrawn amount by design, so this is the
  same rule applied a little harder, and Withdrawal.amount_requested_sats vs
  amount_sent_sats already existed to record the difference.
- bets don't: the bet is a fixed price that can't be quietly reduced. A balance
  exactly equal to the bet is refused with balance_leaves_no_change (translated
  into all 7 languages, carrying required_extra_sats), which turns "a user's
  balance must never exactly equal the bet" from a documented assumption into an
  enforced one — and stops an unbumpable bet from holding a round open until the
  reconciler gives up on it.

bump_fee's no-change guard stays: a single-output tx broadcast before this
change can still be pending across the deploy, and it must fail loudly rather
than start shrinking a recipient's output. Its test now hand-builds that shape,
precisely because the builder no longer will.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -8,7 +8,7 @@ The user communicates in Italian in chat — reply to them in Italian. Everythin
 
 ## Project status
 
-All 10 stages of the original build order are code-complete and unit-tested — 309 tests, all under `tests/unit/` (`tests/integration/` is an empty package). Beyond them: Docker + Caddy deployment, admin dashboard (`/admin`), static test UI (`/`), pending-inclusive balance display, an SSE push channel layered over the original polling, self-service password change + admin password reset, and the reconciliation/corroboration machinery below.
+All 10 stages of the original build order are code-complete and unit-tested — 313 tests, all under `tests/unit/` (`tests/integration/` is an empty package). Beyond them: Docker + Caddy deployment, admin dashboard (`/admin`), static test UI (`/`), pending-inclusive balance display, an SSE push channel layered over the original polling, self-service password change + admin password reset, and the reconciliation/corroboration machinery below.
 
 Verified on mainnet with real money: registration + address derivation, deposit crediting (1-conf), a real 10 PLM bet (broadcast → confirmed → change credited back), and one full round cycle (close → draw on a real block hash → 70/30 payout with sat math checked against the broadcast tx → confirmation → close → next round auto-opened). **Withdrawal and the RBF bump path have never been exercised against a live broadcast** — unit-tested only.
 
@@ -33,7 +33,7 @@ PYTHONPATH=. python scripts/decrypt_master_key.py     # ops recovery: decrypt+pr
 PYTHONPATH=. python scripts/encrypt_master_key.py     # ops bootstrap: import an externally-generated xprv (--overwrite to replace)
 PYTHONPATH=. python scripts/electrum_smoke_test.py    # manual check: connect, handshake, subscribe to headers, print the tip
 
-python -m pytest                                      # all 309 tests
+python -m pytest                                      # all 313 tests
 python -m pytest tests/unit/test_hd.py                # one file
 python -m pytest tests/unit/test_hd.py::test_derivation_is_deterministic   # one test
 ```
@@ -152,7 +152,7 @@ Diagrams: [platform-overview.mmd](flowchart/platform-overview.mmd), [round-lifec
 
 **DEP** — the listener subscribes to the user's scripthash; balance is credited after **1 confirmation** and only once the other servers corroborate the outpoint and its amount (B-59), with the 1-conf reorg risk knowingly accepted and no rollback logic. `deposits/service.py` also detects UTXOs that vanished (spent outside the platform — corroborated per B-29 first) and *reinstates* ones that reappear.
 
-**PLAY** — fixed cost, **at most one active bet per user**, and **at most `MAX_PARTICIPANTS_PER_ROUND` (400) players per round** — past that the bet is refused with `round_full` and the player waits for the next round (B-52: the payout must spend one pool UTXO per bet, so a round is only ever allowed to grow to what a single payout tx can drain). PSBT user-address → pool-address, always with a **change output back to the same user address** (a user's balance must never exactly equal the bet). Fee ~1 sat/vB, **deducted from the bet amount**. No confirmation within the timeout → RBF bump and rebroadcast.
+**PLAY** — fixed cost, **at most one active bet per user**, and **at most `MAX_PARTICIPANTS_PER_ROUND` (400) players per round** — past that the bet is refused with `round_full` and the player waits for the next round (B-52: the payout must spend one pool UTXO per bet, so a round is only ever allowed to grow to what a single payout tx can drain). PSBT user-address → pool-address, always with a **change output back to the same user address** of at least `DUST_LIMIT_SATS` — a user's balance must never exactly equal the bet, and since B-62 that's enforced (`balance_leaves_no_change`) rather than assumed. Fee ~1 sat/vB, **deducted from the bet amount**. No confirmation within the timeout → RBF bump and rebroadcast.
 
 **DRAW** — configurable timer (default 600s):
 - *Bet cutoff is the round's own deadline* (`round_deadline` = `opened_at + Round.duration_seconds`, the value snapshotted at open time — B-61), **not** the DB status: `place_bet` calls `rounds/service.round_accepts_bets`, which rejects once the deadline passes even while `status` is still `"open"` (the 5s scheduler tick can lag behind it). Once a round leaves `open`, no new bets either, and no new round opens until this one is fully `closed`. The deadline is checked twice — on arrival and again after the transaction is built — and the participant row is then committed behind a **compare-and-set on the round row** (`UPDATE rounds ... WHERE status = 'open'`, B-53): the scheduler flips `open` → `closing` in a transaction of its own and only counts in-flight bets afterwards, so without the CAS a bet could commit in between, be excluded from the draw (only `confirmed` participants are drawn) and still have its sats land in the pool with no refund path. Its mirror image on the scheduler side is `_close_and_draw` re-counting in-flight bets in the same session it snapshots the participants from.
@@ -162,7 +162,7 @@ Diagrams: [platform-overview.mmd](flowchart/platform-overview.mmd), [round-lifec
 - *UI, two independent layers.* A generic phase box ("Pagamento al vincitore in corso…") shows to **every** viewer for the whole closing/drawing/paying_out span — pure cosmetic text driven by `status`. **Additively**, a personalized "Hai vinto!/Non hai vinto" box appears only where `user_played` is true (computed via `get_optional_user`, since the endpoint is reachable logged-out) — nobody else has anything to reveal.
 - *Reveal timing.* Delayed by at least `draw_animation_seconds`, anchored to the server's `closes_at` so a reload can't reset the countdown, and decoupled from the real (~block-time) wait for `winner_user_id`. Once revealed it's persisted in `localStorage.plm_persisted_result`, surviving the move to `closed` — at which point `get_active_round` stops returning the round and `winner_user_id` disappears from `GET /rounds/current`. `GET /users/me/last-round-result` is the durable DB-backed backstop for a device that missed the live window entirely. Full logic: `refreshRound`/`checkLastRoundResult` in `app/static/app.js`.
 
-**WITHDRAW** — the only way out to an external address: PSBT user-address → external + change back to the user, fee deducted from the withdrawn amount, same RBF pattern.
+**WITHDRAW** — the only way out to an external address: PSBT user-address → external + change back to the user, fee deducted from the withdrawn amount, same RBF pattern. A full-balance withdrawal moves `balance - DUST_LIMIT_SATS` so the change output (and with it the ability to fee-bump) always exists — `Withdrawal.amount_requested_sats` vs `amount_sent_sats` is what records the difference (B-62).
 
 PLAY and WITHDRAW share a **per-user lock** (`tx/locks.py`): a bet-build and a withdrawal-build can never be in flight at once, since both spend the same UTXO set.
 
@@ -241,7 +241,7 @@ Explicit design choices, not derivable from any single file — respect them:
 Accepted **by design** — distinct from the audit findings above (all fixed), which are not duplicated here.
 
 - **`drawing` doesn't resume after a restart.** `_tick()` handles `open`, `closing` and `paying_out` (the last via `_retry_payout_if_due`); nothing re-enters `_wait_for_next_block` after a crash. That wait is unbounded by design (the draw's entropy genuinely depends on a future block) but no longer silent — past `_DRAW_STALL_THRESHOLD_SECONDS` it logs progress and writes a `draw_stalled` audit entry, and `GET /rounds/current`'s `draw_waiting_since` surfaces it live (B-36). Restart-resumption itself remains the last prerequisite for running unattended.
-- **RBF handles one shape only**: a single change output, back to the tx's own sender, big enough to absorb the increase. No extra-input fallback — an exact-amount tx or too-small change raises `RbfError`. Not permanent, though: an unbumpable tx that never confirms is eventually abandoned and its UTXOs released.
+- **RBF handles one shape only**: a single change output, back to the tx's own sender, big enough to absorb the increase. No extra-input fallback, and none would help the case that used to hurt (an amount equal to the whole input total leaves no other UTXO to add) — which is why `build_signed_transaction` now guarantees a change output of at least `DUST_LIMIT_SATS` instead (B-62): a withdrawal for the full balance moves a dust limit less, a bet from a balance equal to the bet is refused with `balance_leaves_no_change`. What's left is a bump whose *delta* exceeds an otherwise-fine change output, which still raises `RbfError`; that tx is eventually abandoned and its UTXOs released.
 - **Withdrawal and RBF bump are unit-tested but never live-broadcast** (failure and rollback branches included — still not a real network).
 - **No user-facing history.** `GET /users/me/last-round-result` covers exactly one case (the reveal backstop above). Admin has `/admin/rounds`, `/admin/pending-transactions`, `/admin/audit-log`; a user has no equivalent — a failed withdrawal leaves a `failed` row they can never see, which argues for closing this.
 - **Admin auth is one shared bearer token** (`ADMIN_TOKEN`) with no per-admin identity: `audit_log` records *what* changed (config edits as `config_updated`, with before/after) but never *who* did it. It gates the user list, privkey export, password resets and history, so a leak is high-blast-radius.