Fix wellness sync crash: serialize intraday_hr as JSON string for psycopg2

psycopg2 treats Python lists as PostgreSQL arrays (bigint[]) rather than JSON,
causing a DatatypeMismatch error on the json/jsonb column. Serializing with
json.dumps() before the raw SQL INSERT fixes the type error.

Also wrap per-day INSERT in try/except+rollback so one bad day doesn't abort
the entire session, and add db.rollback() in tasks.py after sync_wellness
failure so the final status-update commit can always succeed.

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

backend/app/workers/tasks.py

@@ -530,6 +530,7 @@ def sync_garmin_connect_user(user_id: int):
                 )
             except Exception as exc:
                 errors.append(f"wellness: {exc}")
+                db.rollback()  # recover session so the final status commit can succeed
 
         cfg.last_sync_at = datetime.now(timezone.utc)
         cfg.last_sync_status = (