Real SQLite transactions: BEGIN IMMEDIATE and synchronous=FULL (B-88, DAT-74, DAT-52, ARC-07, TX-28, BET-11, DAT-16)

pysqlite's legacy mode emitted BEGIN only at the first DML, so every read
before it ran outside any transaction and a lock wait surfaced as "database is
locked" at an arbitrary later statement; place_bet also mistook that error for
a write-snapshot conflict, which SQLite cannot produce.

- _register_sqlite_pragmas: synchronous=FULL, isolation_level=None and a
  `begin` event that emits BEGIN / BEGIN IMMEDIATE.
- begin_immediate(session): takes the write lock before the first read; called
  first thing in place_bet, request_withdrawal, the scheduler transitions,
  _trigger_payout, the confirmation poller, the reconciler, bump_fee's persist
  phase, deposit crediting / external-spend marking / reinstating, and the
  derivation-index allocation at registration.
- A lock not granted before a broadcast answers service_busy (503, all seven
  languages), nothing built or sent; after a broadcast nothing is undone, the
  rows stay `building` for the reconciler.
- Spec, storico, BUGS.md and CLAUDE.md updated; 37 new tests on a real file DB.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

app/tx/broadcast.py

@@ -9,6 +9,7 @@ from embit.finalizer import finalize_psbt
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
 
+from app.db.base import begin_immediate
 from app.db.models import PendingTransaction, Round, RoundParticipant, User, UtxoEvent, Withdrawal
 from app.electrum.client import ElectrumClient
 from app.rounds.config import get_round_config
@@ -175,6 +176,7 @@ async def bump_fee(
 
     # --- Phase 3: persist the outcome ----------------------------------------------
     async with session_factory() as session:
+        await begin_immediate(session)  # B-88
         pending = await session.get(PendingTransaction, pending_id)
         old_txid = pending.current_txid
         pending.replaced_by_txid = old_txid  # points backwards: what current_txid replaced