feat(admin): wire instance health panel + settings instance (A9/A10)

AdminInstance shows backing-service health (db/redis/ml), read-only source
status (env-configured), and a library reindex with graceful handling when no
indexable source exists. InstanceSettings shows the active instance and offers
switch/sign-out to /connect. Adds getServices/getSources/reindex endpoints.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/api/types.ts

@@ -140,6 +140,16 @@ export interface UploadResponse {
   already_exists: boolean;
 }
 
+/** One backing-dependency probe result (GET /admin/services). `skipped` = the
+ * dependency is optional and not configured (e.g. ML with no ML_SERVICE_URL). */
+export type ServiceStatus = 'ok' | 'down' | 'skipped';
+
+export interface ServicesStatus {
+  database: ServiceStatus;
+  redis: ServiceStatus;
+  ml: ServiceStatus;
+}
+
 export interface StorageFormatBreakdown {
   fileFormat: string;
   trackCount: number;