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_throttle.py

@@ -0,0 +1,122 @@
+"""
+Unit tests for the Garmin request pacer.
+
+Nothing here touches the network, and the interval is zero (see conftest) so
+the accounting is tested without the waiting.
+"""
+import time
+
+import pytest
+
+from services import garmin_throttle as throttle
+
+
+class Recorder:
+    """Stands in for a garminconnect client."""
+
+    def __init__(self):
+        self.calls = []
+        self.display_name = "Tester"
+        self.garth = object()
+
+    def get_user_summary(self, date):
+        self.calls.append(date)
+        return {"date": date}
+
+    def get_sleep_data(self, date):
+        self.calls.append(date)
+        return {}
+
+
+class TestPacedClient:
+    def test_calls_reach_the_wrapped_client(self):
+        inner = Recorder()
+        client = throttle.pace(inner, "u")
+        assert client.get_user_summary("2026-09-01") == {"date": "2026-09-01"}
+        assert inner.calls == ["2026-09-01"]
+
+    def test_every_call_is_counted(self):
+        client = throttle.pace(Recorder(), "u")
+        client.get_user_summary("d")
+        client.get_sleep_data("d")
+        assert client.limiter.used == 2
+
+    def test_non_callables_pass_through(self):
+        """`display_name` is read by the library on every request, and `garth`
+        carries the login flow — neither is an API call."""
+        inner = Recorder()
+        client = throttle.pace(inner, "u")
+        assert client.display_name == "Tester"
+        assert client.garth is inner.garth
+        assert client.limiter.used == 0
+
+    def test_attribute_writes_reach_the_client(self):
+        inner = Recorder()
+        client = throttle.pace(inner, "u")
+        client.display_name = "Someone"
+        assert inner.display_name == "Someone"
+
+    def test_wrapping_twice_does_not_stack_two_waits(self):
+        once = throttle.pace(Recorder(), "u")
+        twice = throttle.pace(once, "u")
+        assert twice is once
+
+    def test_a_missing_method_still_raises_attribute_error(self):
+        client = throttle.pace(Recorder(), "u")
+        with pytest.raises(AttributeError):
+            client.get_something_that_does_not_exist
+
+
+class TestBudget:
+    def test_the_budget_stops_the_run(self):
+        client = throttle.pace(Recorder(), "u", budget=3)
+        for _ in range(3):
+            client.get_sleep_data("d")
+        with pytest.raises(throttle.BudgetExhausted):
+            client.get_sleep_data("d")
+
+    def test_remaining_counts_down(self):
+        client = throttle.pace(Recorder(), "u", budget=5)
+        client.get_sleep_data("d")
+        assert client.limiter.remaining == 4
+
+    def test_a_new_run_resets_the_budget(self):
+        client = throttle.pace(Recorder(), "u", budget=2)
+        client.get_sleep_data("d")
+        client.get_sleep_data("d")
+        throttle.pace(client, "u", budget=2)
+        client.get_sleep_data("d")  # must not raise
+        assert client.limiter.used == 1
+
+    def test_no_budget_means_unlimited(self):
+        client = throttle.pace(Recorder(), "u", budget=0)
+        # 0 is falsy but explicit; only None means unlimited.
+        with pytest.raises(throttle.BudgetExhausted):
+            client.get_sleep_data("d")
+        limiter = throttle.limiter_for("u")
+        limiter.start_run(None)
+        client.get_sleep_data("d")
+        assert limiter.remaining is None
+
+
+class TestSpacing:
+    def test_calls_are_spaced_by_the_interval(self, monkeypatch):
+        monkeypatch.setenv("GARMIN_MIN_INTERVAL_SECONDS", "0.05")
+        client = throttle.pace(Recorder(), f"spacing-{time.monotonic()}")
+        started = time.monotonic()
+        for _ in range(4):
+            client.get_sleep_data("d")
+        # Three gaps between four calls; the first claims a free slot.
+        assert time.monotonic() - started >= 0.05 * 3
+
+    def test_the_interval_is_read_per_call(self, monkeypatch):
+        monkeypatch.setenv("GARMIN_MIN_INTERVAL_SECONDS", "2.5")
+        assert throttle.min_interval() == 2.5
+        monkeypatch.delenv("GARMIN_MIN_INTERVAL_SECONDS")
+        assert throttle.min_interval() == 0.5, "the researched default"
+
+    def test_accounts_are_paced_independently(self):
+        a = throttle.limiter_for("account-a")
+        b = throttle.limiter_for("account-b")
+        assert a is not b
+        assert throttle.limiter_for("account-a") is a