fix: avoid SQLite lock on fast position poll by skipping DB writes

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

db_conn.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 
 import os
 import sqlite3
+import time
 
 DB_PATH = os.path.join(os.path.dirname(os.path.abspath(__file__)), "futures.db")
 
@@ -17,3 +18,27 @@ def connect_db(path: str | None = None) -> sqlite3.Connection:
     except sqlite3.OperationalError:
         pass
     return conn
+
+
+def execute_retry(
+    conn: sqlite3.Connection,
+    sql: str,
+    params: tuple = (),
+    *,
+    retries: int = 6,
+    base_delay: float = 0.05,
+) -> sqlite3.Cursor:
+    """遇 database is locked 时短暂退避重试。"""
+    last_exc: Exception | None = None
+    for attempt in range(retries):
+        try:
+            return conn.execute(sql, params)
+        except sqlite3.OperationalError as exc:
+            if "locked" not in str(exc).lower():
+                raise
+            last_exc = exc
+            if attempt < retries - 1:
+                time.sleep(base_delay * (attempt + 1))
+    if last_exc:
+        raise last_exc
+    raise sqlite3.OperationalError("database is locked")