fix(sync): 限流退避持久化到 DB，调度器与手动同步均尊重退避期

db.py: 修复迁移非幂等——gunicorn 双 worker 并发 init_db 时后到者遇 Duplicate column 会让整个服务起不来，改为吞掉 duplicate column 错误。garmin.py/scheduler.py: 退避期写入 sync_status.rate_limited_until，自动同步与手动同步在退避期内跳过，不再反复撞击 Garmin 把限流窗口越撞越深。

backend/db.py

@@ -422,6 +422,10 @@ MIGRATIONS = {
         # Which part of the sync is running. "0 / 730 天" says nothing about
         # what is actually happening for the several minutes of it.
         ("stage", "VARCHAR(64)"),
+        # When Garmin answered 429 we stand the account down. Persisted so every
+        # gunicorn worker and a restart agree on the cooldown — a process-local
+        # dict let each worker re-hit Garmin and keep the limit alive forever.
+        ("rate_limited_until", "DATETIME"),
     ],
     "health_data": [
         # activity / energy
@@ -492,9 +496,18 @@ def _migrate(cur):
         for name, coltype in columns:
             if name in present:
                 continue
-            # SQLite has no "ADD COLUMN IF NOT EXISTS"; the membership check
-            # above is what keeps this idempotent on both backends.
-            cur.execute(f"ALTER TABLE {table} ADD COLUMN {name} {coltype}")
+            # The membership check above keeps this idempotent for the normal
+            # case. But gunicorn runs 2 workers that both call init_db() on
+            # boot; under that race the loser can decide to add a column the
+            # winner already added (ADD COLUMN commits implicitly and becomes
+            # visible a hair after the loser's probe). Swallow the
+            # duplicate-column error so a concurrent boot can't take the whole
+            # service down. Same guard covers SQLite's "duplicate column name".
+            try:
+                cur.execute(f"ALTER TABLE {table} ADD COLUMN {name} {coltype}")
+            except Exception as e:  # noqa: BLE001 - only "already exists" is safe to ignore
+                if "duplicate column" not in str(e).lower():
+                    raise
 
 
 # --- Public API -------------------------------------------------------------