fix(db): handle ConnectionDoesNotExistError with retry + pool invalidation

[ohld]

Summary

• pool_pre_ping=True catches most stale connections, but there's a race: Postgres can close a connection between the ping and the actual query
• When this happens asyncpg raises ConnectionDoesNotExistError, which was surfacing as unhandled DBAPIError in fetch_all (Sentry 7347427630) and the webhook handler (Sentry 7379710797)

Two-layer fix in src/database.py:

• handle_error event listener marks ConnectionDoesNotExistError as a pool disconnect — SQLAlchemy invalidates that connection and never returns it from the pool again
• Single retry in fetch_one / fetch_all / execute — if the race fires, the retry acquires a fresh healthy connection and the request succeeds transparently

Test plan

• Deploy to production and monitor Sentry: 7347427630 and 7379710797 should stop producing new events
• Verify app logs show no ConnectionDoesNotExistError after deploy
• docker ps + docker logs health check per CLAUDE.md checklist

🤖 Generated with Claude Code

[ohld]

Staff Engineer Review — APPROVED

Code review complete. Two-layer fix is correct:

1. handle_error listener marks ConnectionDoesNotExistError connections as disconnected — pool invalidates them immediately
2. Single retry in fetch_one/fetch_all/execute handles the race where a stale connection slips past pool_pre_ping

On write retry safety: ConnectionDoesNotExistError fires in asyncpg's _check_open() BEFORE the query is sent to Postgres. No double-write risk — the query never executes on the first attempt.

Pre-existing lint + conflicts fixed: 3 E501 violations in unrelated files, ruff formatting in describe_memes.py, and a merge conflict in wrapped.py with the production hotfix. All resolved and CI now passes (lint ✓ + tests ✓).

Minor informational (non-blocking):

• _is_stale_connection_error uses exc.__cause__ — DBAPIError.orig is the documented SQLAlchemy API but both work in practice
• type().__name__ matching is slightly fragile; isinstance() against imported asyncpg class would be more robust

Merging.