Fix Garmin Connect sync to import full history and prevent re-downloads

Activity sync:
- First sync (no last_sync_at) now fetches from 2010-01-01 instead of -30 days,
  importing the full account history rather than only the last month
- Pre-download dedup: check existing activities by start_time before downloading;
  stamps garmin_activity_id on the match so subsequent syncs take the fast path
- process_activity_file stamps garmin_activity_id on duplicate detection for
  the same reason (covers activities imported via bulk export)
- 0.5 s sleep between downloads to avoid Garmin API rate limiting

Wellness sync:
- First sync now covers last 90 days instead of 7

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

backend/app/workers/tasks.py

@@ -93,6 +93,11 @@ def process_activity_file(self, file_path: str, user_id: int, source_type: str,
         ).scalar_one_or_none()
 
         if existing:
+            # Stamp garmin_activity_id if this came from a Garmin Connect sync
+            # so future syncs skip the fast-path dedup and don't re-download.
+            if garmin_activity_id and not existing.garmin_activity_id:
+                existing.garmin_activity_id = garmin_activity_id
+                db.commit()
             return {"activity_id": existing.id, "status": "duplicate"}
 
         # Get user max HR for zone calculation