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>

frontend/src/components/admin/AdminSidebar.tsx

@@ -10,6 +10,7 @@ import {
   FolderTree,
   ClipboardList,
   MessageSquareText,
+  LayoutGrid,
   ArrowLeft,
 } from 'lucide-react'
 import { cn } from '@/lib/utils'
@@ -25,6 +26,7 @@ const navItems = [
   { path: '/admin/categories', label: 'Categories', icon: FolderTree },
   { path: '/admin/survey-invites', label: 'Survey Invites', icon: ClipboardList },
   { path: '/admin/survey-responses', label: 'Survey Responses', icon: MessageSquareText },
+  { path: '/admin/gallery', label: 'Gallery', icon: LayoutGrid },
 ]
 
 interface AdminSidebarProps {