BUG: Backend: "Conn is possibly unbound" can break db connections

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

There exists at least one execution path where conn might be used before being assigned.
This may break db connections.

Example - face_clusters.py
- Can break functions like db_delete_all_clusters, db_insert_clusters_batch, db_update_cluster
- Definitely causes issue in db_update_cluster, which will be very hard to debug if not fixed now

## Found unbound conns

### face_clusters.py
- Line 61 ,65, 70, 115, 119, 123

### faces.py
- Line 333, 336, 341

### metadata.py
- Line 85, 89, 95

### face_clusters.py
- Line 622, 627, 633

I would like to work on this issue

### Record

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