fix: 本地限流估算不再拦截同步——一律真实请求 Garmin，仅真实 429 记录冷却并退避

此前 sync_data/start_sync/_connect/scheduler 四处会在请求前按本地
rate_limited_until 估算直接拒绝同步，用户看到'立即同步→被限流'实际是
本地拦截、零请求。若 Garmin 已恢复，陈旧估算会让账号一直闲置。

2026-09-02 起冷却只是信息不是闸门：
- start_sync / sync_data 开头移除本地拒绝，_connect 移除冷却提前抛错，
  scheduler 不再跳过冷却中的账号
- 真实 429（refresh_oauth2 / 拉取中途）仍 _note_rate_limit 写 24h 冷却
  并 stand down，中途退避分支保留用冷却算恢复时间
- 成功收尾 _clear_rate_limit 退休陈旧冷却，避免误导后续诊断
- 测试：blocked→仍会真实请求；stale cooldown→healthy connect 放行；
  真实 429 mid-run 仍记 rate_limited；成功清除冷却（596 passed）

backend/services/scheduler.py

@@ -151,16 +151,10 @@ def sync_all_accounts(days=None, respect_schedule=False):
                                     "reason": "not due"})
                     continue
             d = SYNC_DAYS if days is None else days
-            # Never poke Garmin while it is rate-limiting us — that is exactly
-            # what keeps the limit alive. Respect the persisted cooldown and sit
-            # this tick out.
-            blocked = garmin_svc.rate_limited_until(uid)
-            if blocked and blocked > _now():
-                results.append({
-                    "user": uid, "status": "skipped", "reason": "rate-limited",
-                    "retryAfterSeconds": int((blocked - _now()).total_seconds()),
-                })
-                continue
+            # 2026-09-02: the recorded cooldown is no longer consulted before
+            # a sync — a stale estimate must not keep a healthy account idle,
+            # and only Garmin's live answer (a real 429, handled inside
+            # sync_data) decides whether the throttle is actually closed.
             out = garmin_svc.sync_data(uid, {}, days=d, trigger="auto")
             results.append({"user": uid, "status": out.get("status"),
                             "records": out.get("recordsSynced")})