feat(sync): 全部历史真的是全部，不再截断在两年

730 天是个凭空写死的上限。账号有七年数据的人选「全部历史」，拿到的是
最近两年，而且没有任何提示说剩下的被丢掉了。

* 全部历史现在一直回溯到账号最早的数据：连续 EMPTY_RUN_STOP(120) 天
  完全没有内容就停，所以既不会截断，也不会去问手表存在之前的年份。
  MAX_HISTORY_DAYS(3650) 只是兜底，可用环境变量覆盖。
* 已经存过的日期跳过（最近 3 天除外，它们还在写入中）。这让多年的
  回填变成可续传的：撞上限流停下来，冷却过后再点一次就从断点继续，
  而不是每次都从今天重新爬。
* 选择器补上 3 年 / 5 年。
* 前端把每个范围的实际代价写出来，并说明中断可续。

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

backend/tests/test_garmin_sync.py

@@ -646,7 +646,7 @@ class TestBackgroundSync:
             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
+        assert seen["days"] == garmin_svc.MAX_HISTORY_DAYS
 
 
 class TestRateLimiting:
@@ -766,11 +766,49 @@ class TestSyncWindow:
         pulled = {c[1] for c in client.calls if c[0] == "summary"}
         assert len(pulled) == garmin_svc.DEFAULT_SYNC_DAYS
 
-    def test_full_history_asks_for_the_maximum_window(self, db, user):
-        """`days=0` is 全部历史, not "unset"."""
+    def test_full_history_asks_for_the_maximum_window(self, db, user, monkeypatch):
+        """`days=0` is 全部历史, not "unset" — and not a fixed two years."""
+        monkeypatch.setattr(garmin_svc, "MAX_HISTORY_DAYS", 50)
         client = StubClient()
         garmin_svc.sync_data(user["id"], CREDS, days=0, client=client)
-        assert len([c for c in client.calls if c[0] == "summary"]) == 730
+        assert len([c for c in client.calls if c[0] == "summary"]) == 50
+
+    def test_days_already_stored_are_not_refetched(self, db, user, seed_health):
+        """What makes a multi-year backfill resumable: a run that stopped on a
+        429 must continue where it left off, not start over from today."""
+        seed_health([{"date": day(i), "steps": 4000} for i in range(5, 20)])
+        client = StubClient()
+        garmin_svc.sync_data(user["id"], CREDS, days=20, client=client)
+
+        pulled = sorted(c[1] for c in client.calls if c[0] == "summary")
+        assert pulled == sorted(day(i) for i in range(5)), "only the gap"
+
+    def test_the_most_recent_days_are_always_refetched(self, db, user, seed_health):
+        """Today is still being written to; a stored row for it is not final."""
+        seed_health([{"date": day(i), "steps": 4000} for i in range(0, 10)])
+        client = StubClient()
+        garmin_svc.sync_data(user["id"], CREDS, days=10, client=client)
+
+        pulled = {c[1] for c in client.calls if c[0] == "summary"}
+        assert pulled == {day(i) for i in range(garmin_svc.ALWAYS_REFETCH_DAYS)}
+
+    def test_a_long_backfill_stops_at_the_start_of_the_account(self, db, user,
+                                                              monkeypatch):
+        """全部历史 must not spend thousands of requests on years that predate
+        the watch."""
+        monkeypatch.setattr(garmin_svc, "EMPTY_RUN_STOP", 10)
+        empty = {"totalSteps": None, "restingHeartRate": None,
+                 "averageStressLevel": None, "totalKilocalories": None}
+        client = StubClient(
+            summaries={day(i): empty for i in range(3, 400)},
+            sleeps={day(i): {} for i in range(3, 400)},
+            hrvs={day(i): {} for i in range(3, 400)},
+        )
+        out = garmin_svc.sync_data(user["id"], CREDS, days=365, client=client)
+
+        assert out["recordsSynced"] == 3, "the three days that had data"
+        pulled = len([c for c in client.calls if c[0] == "summary"])
+        assert pulled == 13, f"3 real days + 10 empties, not 365 ({pulled})"
 
     def test_the_sync_endpoint_passes_a_zero_through(
         self, client, auth, user, db, monkeypatch
@@ -785,7 +823,9 @@ class TestSyncWindow:
         monkeypatch.setattr(garmin_svc, "start_sync", record)
         r = client.post("/api/garmin/sync", headers=auth, json={"days": 0})
         assert r.status_code == 202
-        assert seen["days"] == 730, "全部历史 must not fall back to the default"
+        # 0 travels all the way to the service, which walks back to the real
+        # end of the account rather than to a fixed ceiling.
+        assert seen["days"] == 0, "全部历史 must not fall back to the default"
 
     def test_a_429_mid_run_stops_the_sync(self, db, user):
         """Filing a 429 as one more skipped day meant a 730-day backfill kept