Reset database state before replacement workers fork

[knifecake]

When a supervised worker dies, replace_fork() queries the database to recover its claimed jobs. This reopens the PostgreSQL connection and pool after the startup cleanup, and start_process() previously forked before resetting that state in the parent.

Reset database connections and pool caches immediately before each os.fork(). Keep the existing startup, parent, and child cleanup, and preserve pooling configuration.

Add PostgreSQL transaction tests that use real connections and pools and intercept only os.fork(). They verify cleanup at the fork boundary for direct starts and replacements following claimed-job recovery, preservation of pooling options, and successful recovery of the dead worker's job. Both tests failed on the prior implementation with an open psycopg connection at the fork boundary.

Validation:

• Full suite on isolated PostgreSQL 17.4: 166 tests passed (uv run python runtests.py with database URLs on port 55439).
• make lint and commit hooks passed.
• Manual macOS/Python 3.12 smoke test with a real supervisor: killed its idle worker, observed a replacement register and successfully execute a job, then confirmed graceful supervisor exit with status 0.

Related to #48; intentionally leaves it open. This fixes a demonstrated cleanup-ordering gap, not a reproduced macOS SIGSEGV. It does not establish that forking alongside background threads is safe or eliminate every possible source of inherited state.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-06T18:17:35.649071Z	a2fef57	Draft marked ready

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