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>

tests/unit/test_db_base.py

@@ -29,7 +29,7 @@ async def test_register_sqlite_pragmas_enables_wal_and_busy_timeout(sqlite_engin
 
     assert (await _pragma(sqlite_engine, "journal_mode")).lower() == "wal"
     assert await _pragma(sqlite_engine, "busy_timeout") == _SQLITE_BUSY_TIMEOUT_MS
-    assert await _pragma(sqlite_engine, "synchronous") == 1  # NORMAL
+    assert await _pragma(sqlite_engine, "synchronous") == 2  # FULL (B-88, DAT-52)
 
 
 async def test_register_sqlite_pragmas_applies_to_every_new_connection(sqlite_engine):