feat(sync): 每小时后台自动同步 + 手动拉取最新接口

- services/scheduler.py：通过 job_locks 表跨 worker 抢占，
  gunicorn 多进程下一个周期只跑一次；claim 超时 30 分钟自动释放，
  避免 worker 中途挂掉把任务永久卡死
- POST /api/garmin/sync-latest：同步执行，窗口 clamp 到 1..7 天
- GET  /api/garmin/auto-sync：返回上次/下次运行时间
- db.py：注释里的分号会被 SCHEMA.split(";") 截断，改为先剥注释再切分

19 项调度器测试，全量 324 项通过

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/db.py

@@ -128,6 +128,17 @@ CREATE TABLE IF NOT EXISTS personal_records (
   FOREIGN KEY (user_id) REFERENCES users(id)
 );
 
+-- Coordination for work that must happen once per interval regardless of how
+-- many gunicorn workers are running. A worker claims a job by writing its
+-- row, and the others see a fresh claim and stand down. Without this the
+-- hourly sync would fire once per worker.
+CREATE TABLE IF NOT EXISTS job_locks (
+  name VARCHAR(64) PRIMARY KEY,
+  holder VARCHAR(64),
+  claimed_at DATETIME,
+  last_run_at DATETIME
+);
+
 -- Rendezvous for the interactive MFA login.
 -- garth asks for the code through a *blocking* callback, so the login parks in
 -- a background thread while the code arrives in a separate HTTP request that
@@ -334,14 +345,25 @@ def _migrate(cur):
 
 
 # --- Public API -------------------------------------------------------------
+def _statements(schema):
+    """Split a schema script into statements.
+
+    Comments are stripped first: splitting the raw text on ';' would cut a
+    comment that happens to contain one in half and hand the remainder to the
+    database as SQL.
+    """
+    lines = [ln for ln in schema.splitlines() if not ln.strip().startswith("--")]
+    for stmt in "\n".join(lines).split(";"):
+        stmt = stmt.strip()
+        if stmt:
+            yield stmt
+
+
 def init_db():
     conn = _connect()
     try:
         cur = conn.cursor()
-        for stmt in SCHEMA.split(";"):
-            stmt = stmt.strip()
-            if not stmt:
-                continue
+        for stmt in _statements(SCHEMA):
             cur.execute(_adapt_sql(stmt))
         _migrate(cur)
     finally: