Add configurable sync_lookback_days for Garmin Connect

Users can now set how many days back the first sync fetches. -1 syncs all
history back to 2010; any positive value sets a rolling window. Values
over 365 show a rate-limit warning in the UI. The default remains 30 days.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/garmin_connect_sync.py

@@ -67,12 +67,13 @@ def authenticate_garmin(email: str, password_enc: str, token_store: Optional[str
 # ── Activity sync ─────────────────────────────────────────────────────────────
 
 def sync_activities(garmin, user_id: int, since: Optional[datetime],
-                    db, file_store_path: str) -> int:
+                    db, file_store_path: str, lookback_days: int = 30) -> int:
     """
     List activities from Garmin Connect, skip any already in the DB, download
     FIT ZIPs for new ones, and queue them for processing.
 
-    On first sync (since=None) fetches the full account history back to 2010.
+    On first sync (since=None) the start date is determined by lookback_days:
+      -1 → full history back to 2010; N → today minus N days.
     On incremental syncs fetches from one day before last_sync_at.
     Returns the number of new activities queued.
     """
@@ -81,8 +82,12 @@ def sync_activities(garmin, user_id: int, since: Optional[datetime],
     from app.models.user import Activity
     from sqlalchemy import select, func
 
-    # First sync: fetch everything; incremental: one day overlap to catch late uploads
-    start_date = (since - timedelta(days=1)).date() if since else date(2010, 1, 1)
+    if since:
+        start_date = (since - timedelta(days=1)).date()
+    elif lookback_days == -1:
+        start_date = date(2010, 1, 1)
+    else:
+        start_date = date.today() - timedelta(days=max(lookback_days, 1))
     end_date = date.today()
 
     try: