[Refactor] Remove legacy Fast conversation storage

[roomote-roomote]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote-roomote or in the web UI.

What changed

• Remove the phase-one Fast conversation bridge triggers, trigger functions, and bridge-only advisory serialization gate before contracting the legacy schema.
• Drop fast_agent_conversation_aliases, slack_quick_answers, and both N-1-only slack_quick_answer_id foreign-key columns.
• Require canonical conversation ownership for Fast integration audit rows and remove the retired schema exports, types, and bridge-era tests.
• Add migration coverage that executes the contraction against a phase-one-shaped schema and verifies the ordered removal.
• Gate Upgrade Compatibility exceptions on the complete 0.41.0 phase-one schema and exact canonical foreign key, allowing only these retired objects while preserving all other schema and cold-restart checks.

Why this change was made

This completes phase 2 of the compatibility removal planned in #1568 after canonical Fast conversation storage shipped in 0.41.0. The legacy schema and write bridge were retained for one release solely to preserve rollout and N-1 rollback safety.

Impact

There is no intended user-facing behavior change. Fast conversation persistence remains canonical, while the database no longer carries the retired compatibility tables or synchronization machinery. Upgrade Compatibility still rejects older or malformed baselines and still requires the supported rollback image to remain healthy through a cold restart on the contracted schema.

Release requirement: this migration must ship only in a release whose supported N-1 rollback target is 0.41.0 or newer. Rolling back to an older release would run code that still reads and writes the schema removed here.

[roomote-community]

No code issues found. See task

_{Reviewed bf027e4}