Add reset-ts endpoint to reset last_sync_ts without clearing sync_map

Needed to safely recover missing docs: run reconcile first (populates
sync_map for existing docs), then reset-ts, then sync (only missing
docs get uploaded, existing ones get metadata patch only).

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

app/api/replicas.py

@@ -176,6 +176,18 @@ def unsuspend_replica(replica_id: int, session: Session = Depends(get_session)):
     return _serialize(replica)
 
 
+@router.post("/{replica_id}/reset-ts")
+def reset_replica_sync_ts(replica_id: int, session: Session = Depends(get_session)):
+    """Reset last_sync_ts to None so the next sync fetches all master docs. Does NOT touch sync_map."""
+    replica = session.get(Replica, replica_id)
+    if not replica:
+        raise HTTPException(404)
+    replica.last_sync_ts = None
+    session.add(replica)
+    session.commit()
+    return {"ok": True, "replica_id": replica_id}
+
+
 @router.post("/{replica_id}/resync")
 async def resync_replica(replica_id: int, session: Session = Depends(get_session)):
     """Wipe sync_map, reset last_sync_ts, and trigger a full resync."""