修复持仓轮询时 SQLite database is locked 错误。

单连接复用并提交风控写入，启用 WAL 与 busy_timeout，缓存风控表 schema 初始化。

Co-authored-by: Cursor <cursoragent@cursor.com>

app.py

@@ -158,8 +158,13 @@ def expire_old_plans():
 # —————————————— 设置读写 ——————————————
 
 def get_db():
-    conn = sqlite3.connect(DB_PATH)
+    conn = sqlite3.connect(DB_PATH, timeout=30)
     conn.row_factory = sqlite3.Row
+    conn.execute("PRAGMA busy_timeout=30000")
+    try:
+        conn.execute("PRAGMA journal_mode=WAL")
+    except sqlite3.OperationalError:
+        pass
     return conn