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/alembic/versions/b8d2f4a6c091_drop_file_uploads_session_id_fk.py

@@ -0,0 +1,34 @@
+"""drop file_uploads session_id foreign key constraint
+
+Revision ID: b8d2f4a6c091
+Revises: a7c9e3b1f402
+Create Date: 2026-03-20 00:00:00.000000
+
+The session_id column on file_uploads previously referenced ai_sessions.id.
+Removing the FK allows the column to reference either AI sessions or regular
+sessions without a constraint violation, while keeping the index for query
+performance.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'b8d2f4a6c091'
+down_revision: Union[str, None] = 'a7c9e3b1f402'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.drop_constraint('file_uploads_session_id_fkey', 'file_uploads', type_='foreignkey')
+
+
+def downgrade() -> None:
+    op.create_foreign_key(
+        'file_uploads_session_id_fkey',
+        'file_uploads', 'ai_sessions',
+        ['session_id'], ['id'],
+        ondelete='SET NULL',
+    )