feat: DB-first architecture with lock-safe writes

- price_monitor: writes live prices to both JSON and mofin.db (holdings + live_prices + portfolio_summary)
- mofin_db: added execute_with_retry/commit_with_retry with exponential backoff on 'database is locked'
- mofin_db: increased timeout 5s->15s, added PRAGMA busy_timeout=15000
- price_monitor retry loop: fixed break-before-if-ok bug (was not retrying on write failure)
- DB connection: WAL mode + retry decorator for all write operations
- cash sync: preserves DB authoritative cash (JSON cash not pushed to DB)

This is the DB-first version. JSON writes remain for dashboard compatibility.
Next step: remove JSON writes entirely for full DB-only architecture.

data/stocks/09868.json

@@ -725,6 +725,11 @@
       "time": "2026-06-14T08:55:34.460610",
       "content": "- 中国人寿(02628) & 小鹏汽车(09868) — 盈亏比过低，不建议买入",
       "report_id": "cron_watchlist_health_weekly_2026-06-13_20-07-41"
+    },
+    {
+      "time": "2026-07-05T08:55:15.087154",
+      "content": "4. 小鹏汽车(09868) — 跌破止损位，汽车行业竞争格局",
+      "report_id": "cron_watchlist_health_weekly_2026-07-04_20-13-24"
     }
   ]
 }