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/688795.json

@@ -26,6 +26,11 @@
       "content": "• **摩尔线程(688795) 616.38** +2.73% | 买入区580-600，偏高→等回调至600以下",
       "report_id": "cron_d3797d924ddc_2026-06-02_16-33-11"
     },
+    {
+      "time": "2026-07-06T08:55:01.761812",
+      "content": "2. **收紧输出质量** — 使用\"关注/观望/留意\"时须紧随具体价格、止损位和触发条件。满 3 次模糊表述自动标记为低质量输出。对 01478 和 688795 做一次明确的持仓策略复核：继续持有",
+      "report_id": "cron_e02b8bde74f8_2026-07-05_22-05-42"
+    },
     {
       "time": "2026-06-14T08:55:34.460610",
       "content": "**🟢 摩尔线程-U(688795) ¥610.55 | 买入区598.34~622.76 ✅ | RR:3.44**",