feat: audit log, device tags, maintenance windows (v0.11.3 Beta)

- Audit log: middleware captures all write operations, paginated admin UI in Settings
- Device tags: full CRUD, device assignment, filter bar and chips on Devices page
- Maintenance windows: alert suppression during planned downtime, Settings management UI

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/types/index.ts

@@ -69,6 +69,14 @@ export interface Device {
   rack_slot?: string;
   created_at: string;
   updated_at?: string;
+  tags?: { id: number; name: string; color: string }[];
+}
+
+export interface Tag {
+  id: number;
+  name: string;
+  color: string;
+  device_count?: number;
 }
 
 export interface Interface {