Corroborate deposit credits, not just external spends (B-59)

A candidate external spend has needed a quorum since B-29, but `value` and
`height` for a *credit* came from the single active connection and went straight
into utxo_events. One hostile or broken server could therefore inflate a user's
displayed balance with outpoints that don't exist. It never spends anyone else's
coins — a bet or withdrawal built on a phantom UTXO is refused at broadcast and
rolled back — but it wedges the balance display and burns build attempts, and on
a custodial platform a balance that isn't real is a support incident either way.
Balances move in both directions; both directions now need the same quorum.

corroborate_utxo_credit asks the other configured servers whether they report
the same outpoint, for the same amount, confirmed. The height itself isn't
compared: a server still catching up reports height 0 and simply doesn't agree,
which is the same answer, while two honest servers can't disagree on the height
of a genuinely confirmed outpoint.

refresh_user gains the phase that shape already implied: find_new_credit_
candidates (new, confirmed, not already held) inside the first session,
corroboration outside any session, then credit_confirmed_utxos over what
survived. Only new outpoints are corroborated — re-checking what we already hold
would open a connection to every other server on every refresh for an answer
that can no longer change anything.

A failed corroboration delays a credit, it never loses one: the next scripthash
notification or DepositReconciler sweep (300s) re-offers the same outpoint, and
the withholding is logged.

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 — 296 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 — 302 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 296 tests
+python -m pytest                                      # all 302 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
 ```
@@ -140,7 +140,7 @@ One connection serves everything — deposit credits, broadcasts, confirmations,
 - **Bounded requests** (`_REQUEST_TIMEOUT_SECONDS` = 15s); a timeout tears the connection down. Unbounded waits used to hang `POST /bets` *while holding the per-user lock*, and could stall the confirmation poller permanently.
 - **The drop is observable**: `client.wait_closed()` resolves when the read loop dies, and `_run_once` races it against the notification consumers and a 60s `server.ping`. Without it the listener sat on queues nobody would ever fill while `listener.client` still looked alive.
 - **Headers are validated, not trusted** (`_apply_header`): the tip never regresses, a header must meet the difficulty target it claims, and a single-block advance must chain from the current tip's hash. Failure raises `HeaderValidationError`, which ends the session like a dropped connection and rotates away — that header is the draw's only entropy, so a fabricated one picks the winner.
-- **A quorum corroborates the two money-moving decisions** (`_corroborate_majority`, 10s per server, asking only the *other* endpoints — never the active one, which is what a MITM controls): `corroborate_header` before a block seeds the draw (B-28), `corroborate_utxo_spent` before a UTXO missing from one `listunspent` is written off as externally spent (B-29). No fallbacks configured → returns True (the accepted risk of an empty `ELECTRUM_FALLBACK_SERVERS`); nobody answers → returns **False**, since an unreachable network proves nothing.
+- **A quorum corroborates every money-moving decision** (`_corroborate_majority`, 10s per server, asking only the *other* endpoints — never the active one, which is what a MITM controls): `corroborate_header` before a block seeds the draw (B-28), `corroborate_utxo_spent` before a UTXO missing from one `listunspent` is written off as externally spent (B-29), and `corroborate_utxo_credit` before a new outpoint credits a balance — same outpoint, same amount, confirmed (B-59). Balances move in both directions, so both directions need the same quorum. No fallbacks configured → returns True (the accepted risk of an empty `ELECTRUM_FALLBACK_SERVERS`); nobody answers → returns **False**, since an unreachable network proves nothing. A failed credit corroboration only *delays*: `find_new_credit_candidates` re-offers the outpoint on the next refresh or `DepositReconciler` sweep.
 
 On reconnect `_subscribe_all_users` runs as its own task with bounded concurrency (`_RESUBSCRIBE_CONCURRENCY` = 20) rather than inline and serially — otherwise a large user base froze `tip_height`, and with it an in-flight draw, for the whole sweep (B-31); one user's failure is logged and skipped. `address_for_new_user` (called right after registration) is best-effort by design: on failure that address stays unsubscribed until the next reconnect or `DepositReconciler` sweep.
 
@@ -150,7 +150,7 @@ Diagrams: [platform-overview.mmd](flowchart/platform-overview.mmd), [round-lifec
 
 **REG** — on signup the server derives a P2WPKH address via BIP84 (`m/84'/coin'/0'/0/index`, one index per user) from the encrypted master xprv. Permanent, and doubles as deposit address, winnings address and withdrawal change address.
 
-**DEP** — the listener subscribes to the user's scripthash; balance is credited after **1 confirmation**, 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.
+**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.