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/models/file_upload.py

@@ -21,7 +21,7 @@ class FileUpload(Base):
         UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=False
     )
     session_id: Mapped[Optional[uuid.UUID]] = mapped_column(
-        UUID(as_uuid=True), ForeignKey("ai_sessions.id", ondelete="SET NULL"), nullable=True, index=True
+        UUID(as_uuid=True), nullable=True, index=True
     )
     filename: Mapped[str] = mapped_column(String(255), nullable=False)
     content_type: Mapped[str] = mapped_column(String(100), nullable=False)