refactor(connections): PostgresConnectionException is defined but never used

[ZhuchkaTriplesix]

lib/core/database/postgres_connection.dart declares PostgresConnectionException, but connect() and other methods only rethrow raw driver exceptions.

Impact: No consistent exception type for callers to catch or classify; UI and telemetry cannot distinguish between SSL, auth, network, and query errors at the connection layer.

Expected: Wrap connection-level failures in PostgresConnectionException with a categorized cause or original exception attached.

Acceptance criteria:

• Throw PostgresConnectionException from connect() and testConnection() on failure.
• Preserve original exception and stack trace.
• Update callers and tests.