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/688981.json

@@ -151,6 +151,11 @@
       "content": "② 中芯国际A(688981) 已跌破止损位！仓位5.44%",
       "report_id": "cron_d42f2ce3b479_2026-07-03_20-03-44"
     },
+    {
+      "time": "2026-07-06T08:55:01.761812",
+      "content": "| **中芯国际双持(688981+00981)** | A+H 双持合计约 9.64% 仓位，两市场同步净多头，若港股中芯继续走弱（注意 00981 止损 ¥77.55 高于成本 ¥75.94 的异",
+      "report_id": "cron_e02b8bde74f8_2026-07-05_22-05-42"
+    },
     {
       "time": "2026-06-11T08:55:23.441938",
       "content": "• 中芯国际(688981) 竞价125.00(-1.81%)，策略买入区116~136内",