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/common/RichTextInput.tsx

@@ -73,10 +73,13 @@ export function RichTextInput({
             })
           })
           .catch((err) => {
+            const errorMsg = err?.response?.status === 503
+              ? 'File uploads not available — contact your administrator'
+              : err?.message || 'Upload failed'
             setPendingUploads((prev) =>
               prev.map((u) =>
                 u.id === upload.id
-                  ? { ...u, status: 'error' as const, error: err?.message || 'Upload failed' }
+                  ? { ...u, status: 'error' as const, error: errorMsg }
                   : u
               )
             )
@@ -196,10 +199,13 @@ export function RichTextInput({
           })
         })
         .catch((err) => {
+          const errorMsg = err?.response?.status === 503
+            ? 'File uploads not available — contact your administrator'
+            : err?.message || 'Upload failed'
           setPendingUploads((prev) =>
             prev.map((u) =>
               u.id === uploadId
-                ? { ...u, status: 'error' as const, error: err?.message || 'Upload failed' }
+                ? { ...u, status: 'error' as const, error: errorMsg }
                 : u
             )
           )