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/pages/FlowPilotAnalyticsPage.tsx

@@ -189,7 +189,7 @@ export default function FlowPilotAnalyticsPage() {
       </div>
 
       {/* Tab bar */}
-      <div className="flex gap-1 border-b border-border">
+      <div className="flex gap-1 border-b border-border overflow-x-auto">
         {TABS.map((tab) => (
           <button
             key={tab.id}