fix: Request-scoped session exposes no post-commit hook, so module caches refresh against uncommitted state

[antosubash]

Closes#268

Headless Pi worker

This is a draft pull request created from an issue carrying the approval label.
It is never merged automatically and requires human review.

Agent summary

Summary

• Added RequestSession.on_commit(callback) supporting sync and async callbacks.
• Callbacks run only after successful commits; rollback/read-only/failed commits discard them.
• Callback failures are logged without affecting committed responses or later callbacks.
• Exported the new session/callback types and documented usage.

Changed areas

• DB session/finalization runtime
• Framework DB public API
• Session documentation and changelog
• Transaction/session tests

Verification

• uv run pytest framework/db/tests — 69 passed
• Ruff format/check — passed
• uv run ty check ... — passed
• uv run python scripts/check_file_size.py — passed
• git diff --check — passed

Visual evidence: none required.

Risks/blockers: none.

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[antosubash]

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