Fix Close Private Repo Connection on Error

[hayesZach]

Closes the private repository connection on initialization error. Also adds function to manually close the connection.

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Note

Improves lifecycle management of the private DB repository and wires the key repo.

• Adds Close() to PrivateRepository and implements graceful DB shutdown in private/private.go
• Cleans up on initialization errors: on any init failure, closes the DB, marks repository as closed, and returns the joined error
• Prevents reuse after close: guards access with RW mutex and returns an error if the repository is already closed
• Implements Key() to return NewKeyRepository (was previously a stub)
• Minor refactors: shared init error handling, added imports, and state (closed, err)

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