Add backup discovery tab with support for local and storage backups

- Add Backup model to Prisma schema with fields for container_id, server_id, hostname, backup info
- Create backupService with discovery methods for local (/var/lib/vz/dump/) and storage (/mnt/pve/<storage>/dump/) backups
- Add database methods for backup CRUD operations and grouping by container
- Create backupsRouter with getAllBackupsGrouped and discoverBackups procedures
- Add BackupsTab component with collapsible cards grouped by CT_ID and hostname
- Integrate backups tab into main page navigation
- Filter storages by node hostname matching to only show applicable storages
- Skip PBS backups discovery (temporarily disabled)
- Add comprehensive logging for backup discovery process

prisma/schema.prisma

@@ -41,6 +41,7 @@ model Server {
   ssh_key_path       String?
   key_generated      Boolean?           @default(false)
   installed_scripts  InstalledScript[]
+  backups            Backup[]
   
   @@map("servers")
 }
@@ -95,3 +96,22 @@ model LXCConfig {
   
   @@map("lxc_configs")
 }
+
+model Backup {
+  id                Int       @id @default(autoincrement())
+  container_id      String
+  server_id         Int
+  hostname          String
+  backup_name       String
+  backup_path       String
+  size              BigInt?
+  created_at        DateTime?
+  storage_name      String
+  storage_type      String    // 'local', 'storage', or 'pbs'
+  discovered_at     DateTime  @default(now())
+  server            Server    @relation(fields: [server_id], references: [id], onDelete: Cascade)
+  
+  @@index([container_id])
+  @@index([server_id])
+  @@map("backups")
+}