fix(garmin): 刷新出来的令牌从来没写回库，于是每次连接都重换一次

「又被限流了」的根因找到了，不是请求量，是令牌。

`_connect` 里 `refresh_oauth2()` 换来的新 OAuth2 令牌只活在进程内存里——
`save_token` 只在绑定账号时调用过一次。于是每次客户端缓存过期（15 分钟）、
每个 gunicorn worker、每次部署重启，都从库里读回**同一个已过期的令牌**，
然后再做一次真实 SSO 换令牌。而 SSO 端点是按账号限流最狠的那个，社区报告能
封 48 小时（garth #217、python-garminconnect #337）。我今天为了部署重启了
八次服务，每次都清掉缓存。

- `refresh_oauth2()` 成功后 `_persist_token()` 写回。拆出这个函数是因为它和
  `save_token` 想要的正好相反：重新绑定要作废现有会话，持久化刷新结果必须
  保住刚刚产出它的那个会话
- 写回时不带 garmin_email，否则 upsert 会把绑定邮箱刷成 NULL，数据同步页会
  忘记绑的是哪个账号
- 刷新加进程内锁，并在拿到锁后重读一次库：另一个线程刚换过就直接用它的，
  不再自己去换一次
- 五条测试盯住这个不变量，包括「冷缓存不该再换一次」（这条如果回归，就是同一
  个 bug 再来一遍）

顺带把数据端点也节流了——那是另外一半问题，不是这次的病因，但一天历史要 9 次
调用，730 天全历史 6600 个请求全速打出去，不该指望佳明一直容忍：

- services/garmin_throttle.py：代理包住 client，所有调用（含以后新加的）都经
  同一个收口，按间隔排队并计数
- 0.5s 是查过的：garmin-data-export 默认 0.15s、garmin-connect-scraper 默认
  3s、官方合作方 API 100 次/分钟（0.6s）。依据写在文件顶部
- 单次同步 1200 个请求预算，跑满就干净收尾、下次接着跑（已存的天数本来就跳过）
- 运动详情每次最多补 40 条——新账号几百条，不限量就是一次性打光预算

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/tests/test_garmin_sync.py

@@ -999,3 +999,132 @@ class TestSyncHistory:
         r = client.get("/api/garmin/sync-history", headers=auth)
         assert r.status_code == 200
         assert r.get_json() == {"items": []}
+
+
+class TestRefreshedTokenIsKept:
+    """The account's repeated lockouts came from here.
+
+    `refresh_oauth2()` mints a token against Garmin's SSO endpoint — the one
+    that limits per account and blocks for hours. The refreshed token used to
+    live only in the process's memory, so every cold client cache (a 15-minute
+    timeout, a second gunicorn worker, a deploy restart) re-read the same
+    expired token from the database and minted another one.
+    """
+
+    @staticmethod
+    def _garmin(refreshes, expired_after_load=True):
+        class StubOAuth2:
+            def __init__(self, expired):
+                self.expired = expired
+
+        class StubGarth:
+            def __init__(self):
+                self.oauth2_token = None
+                self.profile = {"displayName": "Tester"}
+                self.dumped = "refreshed-token"
+
+            def configure(self, **kwargs):
+                pass
+
+            def loads(self, token):
+                self.oauth2_token = StubOAuth2(
+                    expired_after_load if token == "stored-token" else False
+                )
+
+            def refresh_oauth2(self):
+                refreshes.append(1)
+                self.oauth2_token = StubOAuth2(False)
+
+            def dumps(self):
+                return self.dumped
+
+        class StubGarmin:
+            def __init__(self, is_cn=False):
+                self.garth = StubGarth()
+                self.display_name = None
+
+        return StubGarmin
+
+    def test_the_refreshed_token_is_written_back(self, db, user, monkeypatch):
+        refreshes = []
+        monkeypatch.setattr(garmin_svc, "_import_garmin",
+                            lambda: self._garmin(refreshes))
+        garmin_svc.save_token(user["id"], "stored-token", "a@example.com")
+
+        garmin_svc._connect({}, user["id"])
+
+        assert refreshes == [1]
+        assert garmin_svc.load_token(user["id"]) == "refreshed-token", (
+            "a refresh that is not persisted makes the next connect mint "
+            "another token against the endpoint that blocks accounts"
+        )
+
+    def test_persisting_a_refresh_keeps_the_bound_email(self, db, user, monkeypatch):
+        refreshes = []
+        monkeypatch.setattr(garmin_svc, "_import_garmin",
+                            lambda: self._garmin(refreshes))
+        garmin_svc.save_token(user["id"], "stored-token", "a@example.com")
+        garmin_svc.forget_client(user["id"])
+
+        garmin_svc._connect({}, user["id"])
+
+        assert garmin_svc.get_remembered_email(user["id"]) == "a@example.com", (
+            "the 数据同步 page shows this; a refresh must not blank it"
+        )
+
+    def test_a_cold_cache_does_not_refresh_again(self, db, user, monkeypatch):
+        """The whole point: the second connect reads a *valid* stored token."""
+        refreshes = []
+        monkeypatch.setattr(garmin_svc, "_import_garmin",
+                            lambda: self._garmin(refreshes))
+        garmin_svc.save_token(user["id"], "stored-token", "a@example.com")
+
+        garmin_svc._connect({}, user["id"])
+        garmin_svc.forget_client(user["id"])   # as a restart or TTL would
+        garmin_svc._connect({}, user["id"])
+
+        assert refreshes == [1], f"minted {len(refreshes)} SSO tokens, expected 1"
+
+    def test_persisting_the_refresh_does_not_drop_the_live_session(
+        self, db, user, monkeypatch
+    ):
+        """`save_token` invalidates the cached client; the refresh path must
+        not, or every connect would throw its own session away."""
+        refreshes = []
+        monkeypatch.setattr(garmin_svc, "_import_garmin",
+                            lambda: self._garmin(refreshes))
+        garmin_svc.save_token(user["id"], "stored-token", "a@example.com")
+
+        first = garmin_svc._connect({}, user["id"])
+        second = garmin_svc._connect({}, user["id"])
+        assert second is first, "the session was cached, not rebuilt"
+
+    def test_a_rate_limited_refresh_still_surfaces_as_rate_limited(
+        self, db, user, monkeypatch
+    ):
+        class StubGarth:
+            oauth2_token = None
+            profile = {"displayName": "Tester"}
+
+            def configure(self, **kwargs):
+                pass
+
+            def loads(self, token):
+                pass
+
+            def refresh_oauth2(self):
+                raise Exception("429 Client Error: Too Many Requests")
+
+            def dumps(self):
+                return "unused"
+
+        class StubGarmin:
+            def __init__(self, is_cn=False):
+                self.garth = StubGarth()
+
+        monkeypatch.setattr(garmin_svc, "_import_garmin", lambda: StubGarmin)
+        garmin_svc.save_token(user["id"], "stored-token", "a@example.com")
+
+        with pytest.raises(garmin_svc.RateLimited):
+            garmin_svc._connect({}, user["id"])
+        assert garmin_svc.rate_limited_until(user["id"]) is not None