Upgrade to psycopg3

[rudokemper]

Goal

Finishes what #99 started.

What I changed and why

• Refactored db_operations to
  • remove dsn as keyword argument
  • Removed finally: conn.close() block as the context manager handles it (https://access.crunchydata.com/documentation/psycopg3/3.1.9/basic/from_pg2.html#with-connection)
  • added conn.commit() after successful operations and conn.rollback() in exception handlers as psycopg3 requires explicit transaction control (https://www.psycopg.org/psycopg3/docs/basic/transactions.html)
• Minor fixes in a few other places (notably postgres_to_csv, alerts_gcs_test) to align with new behavior from psycopg3.

What I'm not doing here

LLM use disclosure

Claude Sonnet 4.5 in Cursor to help implement changes to imports and adjusting some tests across the repo.