Implement PBS authentication support for backup discovery

- Add PBSStorageCredential model to database schema (fingerprint now required)
- Create PBS credentials API router with CRUD operations
- Add PBS login functionality to backup service before discovery
- Create PBSCredentialsModal component for managing credentials
- Integrate PBS credentials management into ServerStoragesModal
- Update storage service to extract PBS IP and datastore info
- Add helpful hint about finding fingerprint on PBS dashboard
- Auto-accept fingerprint during login using stored credentials

src/server/services/storageService.ts

@@ -182,6 +182,20 @@ class StorageService {
     return allStorages.filter(s => s.supportsBackup);
   }
 
+  /**
+   * Get PBS storage information (IP and datastore) from storage config
+   */
+  getPBSStorageInfo(storage: Storage): { pbs_ip: string | null; pbs_datastore: string | null } {
+    if (storage.type !== 'pbs') {
+      return { pbs_ip: null, pbs_datastore: null };
+    }
+    
+    return {
+      pbs_ip: (storage as any).server || null,
+      pbs_datastore: (storage as any).datastore || null,
+    };
+  }
+
   /**
    * Clear cache for a specific server
    */