Issue #2 from phase-8-review-issues.md. Apply was client-side-only via
a bannerApplied flag. Refresh / chat reselect / multi-tab would drop
Verifying state back to Proposed.
- New POST /ai-sessions/{sid}/suggested-fixes/{fid}/apply stamps
applied_at without changing status (still 'proposed'). Idempotent
if already stamped; 409 if fix is past proposed (a terminal outcome
was already recorded).
- Bumps state_version so resolve/escalate preview bundles reflect that
the fix has entered verifying.
- Frontend handleApplyFix calls the endpoint and uses the returned
applied_at directly. bannerApplied client flag is removed.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>