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/688411.json

@@ -861,6 +861,11 @@
       "content": "{\"type\":\"周复盘\",\"time\":\"周日\",\"summary\":\"A股先跌后弹分化格局，全市场普跌后暴力反弹\",\"key_holdings\":[{\"code\":\"600110\",\"name\":",
       "report_id": "cron_e02b8bde74f8_2026-06-14_22-04-57"
     },
+    {
+      "time": "2026-07-06T08:55:01.761812",
+      "content": "2. **海博思创(688411)追踪较好** — 该股在持仓中占比 6.31%，成本 ¥266.95 处于买入区内（¥231.32~¥269.87），参与价合理，后续追踪记录清晰，止盈 ¥277.5",
+      "report_id": "cron_e02b8bde74f8_2026-07-05_22-05-42"
+    },
     {
       "time": "2026-06-03T09:56:06.723399",
       "content": "| 688411 | 海博思创 | 295.00 | 299.17 | -1.39% | 追踪止盈290 | 正常 |",