Fix Bank Connections

Clean up ghost bank connections and reactivate incorrectly expired ones.

Safe to run — only deactivates duplicates and resets connections that were incorrectly marked expired. Does NOT delete any data.

Steps:

1. Reactivate connections marked consent_expired=True whose consent_expires is still in the future.
2. For each user, deduplicate accounts by (sort_code, account_number). Keep the account whose parent connection has the most recent last_synced_at; deactivate the parent connections of the rest.
3. For connections sharing the same finexer_consent_id within a user, keep the most recently synced one and deactivate the rest.