Add types to _get_db

[cdrini]

Note: This might cause more trouble than it's worth, since the webpy DB implementation isn't well-typed. But all the tests are passing...

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Testing

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[Copilot]

Pull request overview

Adds a type cast to _get_db() in openlibrary/core/db.py to make the cached DB accessor appear as a PostgresDB to type checkers (and imports the associated typing helpers).

Changes:

• Import cast and PostgresDB.
• Cast the return value of web.database(**web.config.db_parameters) to PostgresDB.

[Copilot]

_get_db() can return a SQLite-backed web.py DB in unit tests (e.g., tests set web.config.db_parameters = {"dbn": "sqlite", ...}), so casting the result of web.database(...) to PostgresDB is too narrow/misleading and can hide type errors (type checker will allow Postgres-only attributes even though runtime may be SQLite). Use a broader return type (common base/interface/Protocol or Union of supported DB types) and add an explicit return type annotation instead of forcing PostgresDB here.