fix(sync): 限流拦截不再把'上次同步'刷成现在 + 设置页新增立即同步按钮

- _set_sync_status 支持保留旧 last_sync_time；两处限流拦截(start_sync /
  sync_data)改为传旧值——被拒的同步零请求发生，不能把 UI 的'上次同步'
  拨快成'刚刚'，掩盖数据早已停更的事实
- SettingsPage 同步区块新增'立即同步'行：走 /garmin/sync-latest(最近2天，
  秒级阻塞返回)，成功/限流/未绑定三种状态都有明确反馈；限流中后端零请求
  拦截并回传恢复倒计时文案

backend/services/garmin.py

@@ -34,7 +34,15 @@ from services import garmin_extras as extras
 DEFAULT_SYNC_DAYS = int(os.environ.get("GARMIN_SYNC_DAYS") or 7)
 
 
-def _set_sync_status(user_id, status, now, **fields):
+def _set_sync_status(user_id, status, now, last_sync_time=None, **fields):
+    """Write the status row; `now` becomes the new last_sync_time by default.
+
+    Callers that merely *report* being blocked (a rate-limit refusal, a
+    refused start) pass the previous last_sync_time explicitly: a sync that
+    never happened must not move "上次同步" forward, or the UI reads "刚刚"
+    while the data actually stopped syncing hours ago.
+    """
+    last = last_sync_time if last_sync_time is not None else now
     cols = ["user_id", "status", "last_sync_time"] + list(fields.keys())
     placeholders = ", ".join(["?"] * len(cols))
     if DB_TYPE == "mariadb":
@@ -49,7 +57,7 @@ def _set_sync_status(user_id, status, now, **fields):
             f"INSERT INTO sync_status ({', '.join(cols)}) VALUES ({placeholders}) "
             f"ON CONFLICT(user_id) DO UPDATE SET {updates}"
         )
-    execute(sql, [user_id, status, now] + list(fields.values()))
+    execute(sql, [user_id, status, last] + list(fields.values()))
 
 
 def get_sync_status(user_id):
@@ -971,8 +979,15 @@ def start_sync(user_id, creds, days=None):
     now = datetime.datetime.utcnow().isoformat(timespec="seconds")
     until, msg = _rate_limit_block(user_id)
     if until:
+        # A refused start synced nothing — keep the previous last_sync_time so
+        # the UI's 上次同步 keeps telling the truth (the data is stale since
+        # the last *successful* sync, and that date is what the user needs).
+        row = query_one(
+            "SELECT last_sync_time FROM sync_status WHERE user_id = ?", [user_id]
+        )
         _set_sync_status(
             user_id, "rate_limited", now,
+            last_sync_time=row["last_sync_time"] if row else None,
             records_synced=0, progress_current=0, progress_total=days,
             started_at=now, last_error=msg,
         )
@@ -1005,9 +1020,14 @@ def sync_data(user_id, creds, days=None, client=None):
     until, msg = _rate_limit_block(user_id)
     if until:
         # A blocked account must issue zero Garmin requests — that is the whole
-        # point. Return immediately without touching the network.
+        # point. Return immediately without touching the network, and keep the
+        # previous last_sync_time (this was not a sync).
+        row = query_one(
+            "SELECT last_sync_time FROM sync_status WHERE user_id = ?", [user_id]
+        )
         _set_sync_status(
             user_id, "rate_limited", now,
+            last_sync_time=row["last_sync_time"] if row else None,
             records_synced=0, last_error=msg,
         )
         return {