fix(pilot): persist Apply — stamp applied_at on click

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>

backend/app/api/endpoints/session_suggested_fixes.py

@@ -217,6 +217,68 @@ async def record_decision(
     )
 
 
+# ── Suggested fix: apply (stamp applied_at) ──────────────────────────────
+
+@router.post(
+    "/suggested-fixes/{fix_id}/apply",
+    response_model=SessionSuggestedFixResponse,
+)
+async def apply_suggested_fix(
+    session_id: UUID,
+    fix_id: UUID,
+    current_user: Annotated[User, Depends(get_current_active_user)],
+    db: Annotated[AsyncSession, Depends(get_db)],
+    _: None = Depends(require_engineer_or_admin),
+) -> SessionSuggestedFixResponse:
+    """Stamp applied_at when the engineer clicks Apply in the ProposalBanner.
+
+    This does NOT change status (fix remains 'proposed'). Status only flips
+    when the engineer records an outcome via PATCH /outcome.
+
+    Rules:
+    - Fix must be in 'proposed' status; any other status → 409.
+    - Idempotent: if applied_at is already set, returns 200 with the unchanged row.
+    - Bumps ai_sessions.state_version so resolve/escalate preview generators
+      know the fix has entered the verifying phase.
+    """
+    await _load_session_or_404(db, session_id)
+
+    result = await db.execute(
+        select(SessionSuggestedFix).where(
+            SessionSuggestedFix.id == fix_id,
+            SessionSuggestedFix.session_id == session_id,
+        )
+    )
+    fix = result.scalar_one_or_none()
+    if fix is None:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND, detail="Suggested fix not found"
+        )
+
+    if fix.status != "proposed":
+        raise HTTPException(
+            status_code=status.HTTP_409_CONFLICT,
+            detail=f"Apply is only valid from 'proposed'; fix is already '{fix.status}'",
+        )
+
+    # Idempotent: already stamped → return as-is without bumping state_version again.
+    if fix.applied_at is not None:
+        return SessionSuggestedFixResponse.model_validate(fix)
+
+    fix.applied_at = datetime.now(timezone.utc)
+
+    # Bump state_version so preview generators see the verifying-phase signal.
+    await db.execute(
+        update(AISession)
+        .where(AISession.id == session_id)
+        .values(state_version=AISession.state_version + 1)
+    )
+
+    await db.commit()
+    await db.refresh(fix)
+    return SessionSuggestedFixResponse.model_validate(fix)
+
+
 # ── Suggested fix: outcome ────────────────────────────────────────────────
 
 @router.patch(