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/confirmation.py

@@ -5,6 +5,7 @@ from collections.abc import Awaitable, Callable
 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
 from app.electrum.client import ElectrumClient
 from app.electrum.scripthash import address_to_scripthash
@@ -80,6 +81,7 @@ async def poll_once(session_factory: async_sessionmaker, client: ElectrumClient)
             continue
 
         async with session_factory() as session:
+            await begin_immediate(session)  # B-88: read the row and write its outcome under one lock
             row = await session.get(PendingTransaction, pending_id)
             if row is None or row.status != "pending":
                 continue