Database connections are never closed

PgstacDB maintains a persistent connection to the database through self.connection = pool.getconn(). According to the getconn() documentation, this connection will not be automatically returned to the pool, and therefore won't be closed when idle. An explicit putconn() call is required. Currently, db_max_idle is ignored, which leads to database timeouts during long idle periods. I suggest not implementing a custom self.connection singleton pooling logic, but rather relying on the psycopg_pool implementation. Otherwise, you would need to implement the same feature set as psycopg_pool, including idle handling, which adds unnecessary complexity.

Start by tracing PgstacDB's self.connection assignment and the pool.getconn() usage described in the issue, then compare that lifecycle with psycopg_pool's idle handling and putconn() requirements. Done means idle database connections are returned or closed according to db_max_idle and no longer cause timeouts after long idle periods.