[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/services/garmin.py

@@ -21,6 +21,7 @@ so passing it a date silently asks for activity number "2026-08-23".
 """
 import datetime
 import os
+import threading
 
 from db import execute, query_one
 from config import DB_TYPE
@@ -62,6 +63,9 @@ def get_sync_status(user_id):
         "lastSyncTime": row["last_sync_time"],
         "recordsSynced": row["records_synced"],
         "lastError": row["last_error"],
+        "progressCurrent": row.get("progress_current"),
+        "progressTotal": row.get("progress_total"),
+        "startedAt": row.get("started_at"),
     }
 
 
@@ -411,6 +415,31 @@ def _sync_activities(client, user_id, start_date, end_date):
     return stored
 
 
+# Above this many days a sync is long enough that the caller must not block
+# on it — a year takes roughly 20 minutes at ~3s per day.
+BACKGROUND_THRESHOLD_DAYS = 14
+
+
+def start_sync(user_id, creds, days=None):
+    """Run a sync in the background and return immediately.
+
+    Progress lands in sync_status, which the UI polls; a full backfill runs
+    far longer than any sensible HTTP timeout.
+    """
+    days = days or DEFAULT_SYNC_DAYS
+    now = datetime.datetime.utcnow().isoformat(timespec="seconds")
+    _set_sync_status(
+        user_id, "syncing", now,
+        records_synced=0, progress_current=0, progress_total=days,
+        started_at=now, last_error=None,
+    )
+    thread = threading.Thread(
+        target=sync_data, args=(user_id, creds, days), daemon=True
+    )
+    thread.start()
+    return {"status": "syncing", "days": days}
+
+
 def sync_data(user_id, creds, days=None, client=None):
     """Pull the last `days` days from Garmin Connect into the local database.
 
@@ -418,7 +447,10 @@ def sync_data(user_id, creds, days=None, client=None):
     """
     days = days or DEFAULT_SYNC_DAYS
     now = datetime.datetime.utcnow().isoformat(timespec="seconds")
-    _set_sync_status(user_id, "syncing", now, records_synced=0)
+    _set_sync_status(
+        user_id, "syncing", now,
+        records_synced=0, progress_current=0, progress_total=days,
+    )
 
     try:
         client = client or _connect(creds, user_id)
@@ -451,6 +483,15 @@ def sync_data(user_id, creds, days=None, client=None):
             health.upsert_health_daily(user_id, record)
             days_synced += 1
 
+        # Reported every few days rather than every day: the write is cheap
+        # but not free, and the UI polls on a 2s cadence anyway.
+        if (i + 1) % 5 == 0 or i + 1 == days:
+            _set_sync_status(
+                user_id, "syncing", now,
+                records_synced=days_synced, progress_current=i + 1,
+                progress_total=days,
+            )
+
     activities_synced = 0
     try:
         activities_synced = _sync_activities(
@@ -482,6 +523,7 @@ def sync_data(user_id, creds, days=None, client=None):
 
     _set_sync_status(
         user_id, "idle", now, records_synced=days_synced,
+        progress_current=days, progress_total=days,
         last_error="; ".join(day_errors[:3]) if day_errors else None,
     )
     message = (