Consider PostgreSQL advisory locks instead of redis + file locking

### Describe the problem

https://github.com/WeblateOrg/weblate/pull/21445 introduces usage of PostgreSQL advisory locks (because it needed shared locks that the current Redis implementation doesn't offer). This might be actually a good fit for the existing locks as it allows scoping them to transaction without need for artificial timeouts and refreshes.

### Solution brainstorm

Revisit WeblateLock implementation and make it PostgreSQL only and scoped to transaction. No file-based fallback, no refreshes, no timeouts.

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

_No response_

Start by reading the WeblateLock implementation and pull request #21445, focusing on how Redis and file-based locking currently handle shared locks, refreshes, and timeouts. Done means WeblateLock uses PostgreSQL advisory locks scoped to a transaction, with no file fallback, refreshes, or artificial timeouts.

Written by the indexing model from the issue text.