Fix SQLite lock errors on /api/stats under concurrent writes.

Retry stats cache commits, serialize refresh, and fall back to read-only compute so the stats API does not return 500 when the database is briefly locked.

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

db_conn.py

@@ -47,3 +47,26 @@ def execute_retry(
     if last_exc:
         raise last_exc
     raise sqlite3.OperationalError("database is locked")
+
+
+def commit_retry(
+    conn: sqlite3.Connection,
+    *,
+    retries: int = 6,
+    base_delay: float = 0.05,
+) -> None:
+    """遇 database is locked 时短暂退避重试 commit。"""
+    last_exc: Exception | None = None
+    for attempt in range(retries):
+        try:
+            conn.commit()
+            return
+        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")