[阶段7.1] 同步改为后台任务 + 进度上报，支持回补历史

趋势页提供了"一年"档，但库里只有 7 天数据，那一档形同虚设。
实测每天约 2.84 秒（一天要打 5 个端点），回补一年需要约 28 分钟，
远超任何 HTTP 超时能等的时间。

- sync_status 新增 progress_current / progress_total / started_at
- start_sync() 起后台线程并立即返回，sync_data 每 5 天写一次进度
  （写库便宜但不免费，而前端本来就是 2 秒一轮询）
- POST /api/garmin/sync 改为 202 立即返回，接受 days 参数并
  夹在 1..730；进度经 GET /status 轮询
- 一次新同步会清掉上一次的错误，避免旧错误一直挂在界面上

前端:
- 同步页给出 7 / 30 / 90 / 365 天四个选项，日常与首次回补分开
- 进度条显示"第 N / 共 M 天"与预计耗时，并说明可以离开本页
- 页面挂载时若发现正在同步会接着轮询 —— 回补比页面存活时间长，
  刷新后必须能接上进度

tests (+7, 共 299):
- start_sync 在工作完成前就返回，且返回前已把 total 写好
- 进度随同步推进，结束时等于总天数
- days 超范围被夹到 730
- 新同步清除上一次的错误

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

backend/tests/test_garmin_sync.py

@@ -527,3 +527,78 @@ class TestTimestampNormalisation:
 
     def test_nonsense_value_does_not_raise(self):
         assert garmin_svc._to_datetime(float("inf")) is None
+
+
+class TestBackgroundSync:
+    """A full backfill runs for ~20 minutes at ~3s per day, so the request
+    must not block on it and the UI needs progress rather than a spinner."""
+
+    def test_progress_is_reported_during_the_run(self, db, user):
+        garmin_svc.sync_data(user["id"], CREDS, days=10, client=StubClient())
+        status = garmin_svc.get_sync_status(user["id"])
+        assert status["progressTotal"] == 10
+        assert status["progressCurrent"] == 10
+
+    def test_progress_total_matches_the_requested_window(self, db, user):
+        garmin_svc.sync_data(user["id"], CREDS, days=3, client=StubClient())
+        assert garmin_svc.get_sync_status(user["id"])["progressTotal"] == 3
+
+    def test_start_sync_returns_immediately(self, db, user, monkeypatch):
+        import threading
+        release = threading.Event()
+
+        def slow(uid, creds, days=None, client=None):
+            release.wait(5)
+
+        monkeypatch.setattr(garmin_svc, "sync_data", slow)
+        out = garmin_svc.start_sync(user["id"], CREDS, days=365)
+
+        # Returns before the work finishes.
+        assert out["status"] == "syncing"
+        assert out["days"] == 365
+        assert garmin_svc.get_sync_status(user["id"])["status"] == "syncing"
+        release.set()
+
+    def test_start_sync_marks_total_before_any_work(self, db, user, monkeypatch):
+        import threading
+        release = threading.Event()
+        monkeypatch.setattr(
+            garmin_svc, "sync_data", lambda *a, **k: release.wait(5)
+        )
+        garmin_svc.start_sync(user["id"], CREDS, days=200)
+        status = garmin_svc.get_sync_status(user["id"])
+        assert status["progressTotal"] == 200
+        assert status["progressCurrent"] == 0
+        release.set()
+
+    def test_previous_error_is_cleared_when_a_new_sync_starts(
+        self, db, user, monkeypatch
+    ):
+        garmin_svc.sync_data(
+            user["id"], CREDS, days=1, client=StubClient(fail_days=[day(0)])
+        )
+        assert garmin_svc.get_sync_status(user["id"])["lastError"]
+
+        import threading
+        release = threading.Event()
+        monkeypatch.setattr(garmin_svc, "sync_data", lambda *a, **k: release.wait(5))
+        garmin_svc.start_sync(user["id"], CREDS, days=7)
+        assert not garmin_svc.get_sync_status(user["id"])["lastError"]
+        release.set()
+
+    def test_endpoint_returns_202_without_waiting(self, client, auth, user, db, monkeypatch):
+        garmin_svc.save_token(user["id"], "blob", "g@example.com")
+        monkeypatch.setattr(garmin_svc, "start_sync", lambda *a, **k: {"status": "syncing", "days": 30})
+        r = client.post("/api/garmin/sync", headers=auth, json={"days": 30})
+        assert r.status_code == 202
+        assert r.get_json()["status"] == "syncing"
+
+    def test_days_is_clamped_to_a_sane_range(self, client, auth, user, db, monkeypatch):
+        garmin_svc.save_token(user["id"], "blob", "g@example.com")
+        seen = {}
+        monkeypatch.setattr(
+            garmin_svc, "start_sync",
+            lambda uid, creds, days=None: seen.setdefault("days", days) or {"status": "syncing"},
+        )
+        client.post("/api/garmin/sync", headers=auth, json={"days": 99999})
+        assert seen["days"] == 730