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/688639.json

@@ -91,6 +91,11 @@
       "content": "- 华恒生物(688639) ¥21.2，买入区20.78~21.4 ✅ 在区间内，RR:2.28",
       "report_id": "cron_e02b8bde74f8_2026-06-14_22-04-57"
     },
+    {
+      "time": "2026-07-06T08:55:01.761812",
+      "content": "3. **策略数据翻新** — 针对成本远高于止盈的 13 个持仓（如 万科02202、比亚迪01211、华恒生物688639、中科电气300035、黄金ETF518880 等），建议逐个审核是否需要",
+      "report_id": "cron_e02b8bde74f8_2026-07-05_22-05-42"
+    },
     {
       "time": "2026-06-01T10:25:54.503460",
       "content": "华恒生物(688639) 仓位0.47% -1.71%→ 持有，低位整理",