fix: 6 integration audit fixes — ticket filter, admin nav, FK scope, debounce, error messages

- Fix AISession.ticket_id → psa_ticket_id in list_sessions filter query
- Add Gallery nav item (LayoutGrid icon) to AdminSidebar navItems array
- Remove ForeignKey from FileUpload.session_id (Python model) + migration b8d2f4a6c091 to drop DB constraint, allowing column to reference either session type
- Add 400ms debounce on AI session search input in SessionHistoryPage (aiSearchInput state + useRef timeout pattern)
- Show friendly 503 error message in RichTextInput upload error handler (both initial upload and retry paths)
- Add overflow-x-auto to FlowPilotAnalyticsPage tab bar container

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/endpoints/ai_sessions.py

@@ -562,7 +562,7 @@ async def list_sessions(
     if confidence_tier:
         query = query.where(AISession.confidence_tier == confidence_tier)
     if ticket_id:
-        query = query.where(AISession.ticket_id == ticket_id)
+        query = query.where(AISession.psa_ticket_id == ticket_id)
     if date_from:
         query = query.where(AISession.created_at >= date_from)
     if date_to: