Snapshot a round's timing when it opens (B-61)

round_duration_seconds was read live on every scheduler tick and every bet
check, with the deadline computed as opened_at + duration. Lowering it from 600
to 60 while a round was 300s in closed that round instantly; raising it moved
the closes_at clients were already counting down to. round_cooldown_seconds had
the same property for the gap after a close. B-11 fixed this class of problem
for the advertised jackpot; the timing fields were left live.

Round now carries duration_seconds and cooldown_seconds, set from the config
when it opens. round_deadline() is the single place the deadline is computed —
the scheduler, place_bet's two checks and /rounds/current's closes_at all go
through it — and the cooldown is read off the round that just closed, so the gap
a round announced is the gap that's honoured. The config row becomes what the
*next* round opens with.

The migration backfills from the live config rather than leaving the column
defaults: an instance running 300s rounds would otherwise see the round
currently in progress jump to 600s the moment this lands, which is precisely the
retroactive change being fixed. Verified against a scratch DB with a non-default
config.

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 — 305 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 — 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.
 
 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 305 tests
+python -m pytest                                      # all 309 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
 ```
@@ -84,7 +84,7 @@ Source of truth: the `PalladiumWallet` repo — [ChainProfiles.cs](../PalladiumW
 |---|---|---|
 | Bet cost | 10 PLM (`bet_amount_sats = 1_000_000_000`) | `RoundConfig`, admin-editable |
 | Prize split | **70% winner / 30% fees**, rounding remainder to fees | **hardcoded** in `rounds/scheduler.py` — a code change, not an admin edit |
-| Round duration / cooldown | 600s / 30s | `RoundConfig` |
+| Round duration / cooldown | 600s / 30s | `RoundConfig`, but **snapshotted onto `Round.duration_seconds`/`cooldown_seconds` when a round opens** (B-61) — an edit applies from the next round, never to the one in progress |
 | Draw animation | 20s (cosmetic frontend minimum only) | `RoundConfig` |
 | Fee rate / RBF timeout | 1 sat/vB / 900s | `RoundConfig` |
 | Min withdrawal | = current `bet_amount_sats` (no separate field) | `withdrawals/service.py` |
@@ -155,7 +155,7 @@ Diagrams: [platform-overview.mmd](flowchart/platform-overview.mmd), [round-lifec
 **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.
 
 **DRAW** — configurable timer (default 600s):
-- *Bet cutoff is the round's own deadline* (`opened_at + round_duration_seconds`), **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.
+- *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.
 - *"Yellow light":* closing **waits for every already-broadcast bet to confirm** before drawing, so a bet in flight at the boundary isn't lost (`building` counts as in-flight; what bounds the wait is the reconciler eventually abandoning a bet that never confirms).
 - *Algorithm* (deliberately simple, meant to be replaced): first block confirmed after closing — corroborated by the other servers first, and on failure the draw waits for a *further* block and writes a `draw_header_corroboration_failed` audit entry rather than stalling silently — hash as seed, `index = seed mod participant_count` over participants ordered by **broadcast timestamp** (also the tie-break when two bets land in the same block). Equal probability for everyone, regardless of amount.
 - *Payout* is signed with the pool key; its **fee comes out of the winner's 70%**, leaving the 30% fee share intact. Same timeout → RBF → rebroadcast pattern.