完全DB版: 移除JSON写入/回退，所有操作直走SQLite

- price_monitor: 完全DB版，不再读写 portfolio.json/watchlist.json
  - holdings代码从DB读取（SELECT code FROM holdings WHERE is_active=1）
  - 价格写入DB holdings表 + portfolio_summary + live_prices
  - 3次重试+指数退避防锁
- server.py: 移除API的JSON回退，DB失败直接返回500
- mofin_db.py: execute_with_retry/commit_with_retry + 15s timeout + busy_timeout
- mo_data.py: 已是纯DB模式（无JSON fallback）

这是推进的完全DB化。下一步可删除 data/portfolio.json data/decisions.json data/watchlist.json 等遗留JSON。

data/portfolio.json

@@ -5,9 +5,9 @@
       "name": "腾讯",
       "shares": 100,
       "cost": 443.13,
-      "price": 447.0,
+      "price": 446.4,
       "market_value": 39063.0,
-      "change_pct": 3.66,
+      "change_pct": 3.53,
       "currency": "HKD",
       "position_pct": null
     },
@@ -16,9 +16,9 @@
       "name": "中国神华",
       "shares": 500,
       "cost": 45.89,
-      "price": 40.68,
+      "price": 40.72,
       "market_value": 17575.0,
-      "change_pct": 1.7,
+      "change_pct": 1.8,
       "currency": "HKD",
       "position_pct": 2.14
     },
@@ -27,9 +27,9 @@
       "name": "比亚迪股份",
       "shares": 600,
       "cost": 104.87,
-      "price": 84.2,
+      "price": 84.25,
       "market_value": 44376.0,
-      "change_pct": 0.12,
+      "change_pct": 0.18,
       "currency": "HKD",
       "position_pct": 4.62
     },
@@ -38,9 +38,9 @@
       "name": "丘钛科技",
       "shares": 11000,
       "cost": 13.47,
-      "price": 6.77,
+      "price": 6.75,
       "market_value": 64460.0,
-      "change_pct": -3.15,
+      "change_pct": -3.43,
       "currency": "HKD",
       "position_pct": 7.97
     },