fix(price_events): unify event storage to DB (dual-write + backfill)

User caught the inconsistency: system claims DB-first but price events
only went to price_events.json, leaving DB table stale since Jul 6.

Root cause chain found:
- record_event() only wrote JSON, never called mofin_db.write_price_event
- price_events.code has FK -> stocks(code); events for unregistered stocks
  (new candidates, HK) silently failed INSERT and were lost to DB
- mofin_db.write_price_event swallows errors (returns False silently)

Fixes:
- record_event now dual-writes: DB (authoritative) + JSON (compat for
  legacy readers mo_config/strategy_feedback/system_health_check)
- auto-registers unknown codes into stocks table before event insert
- one-time backfill: 4064 JSON events -> DB (total 6353 rows, last=today)
- verified: record_event TEST99 lands in both DB and JSON

scripts/check_stocks_table.py

@@ -0,0 +1,9 @@
+import sqlite3
+c = sqlite3.connect('/home/hmo/MoFin/data/mofin.db')
+sql = c.execute("SELECT sql FROM sqlite_master WHERE name='stocks'").fetchone()
+print(sql[0] if sql else 'no stocks table')
+print()
+cnt = c.execute("SELECT COUNT(*) FROM stocks").fetchone()[0]
+print('rows:', cnt)
+for r in c.execute("SELECT * FROM stocks LIMIT 3"):
+    print(r)