Add missing BackupMonitor methods for health check endpoints

- Add get_recent_backups method to return recent backup history
- Add get_failed_backups method to return failed backup history
- Add get_container_backup_history method for container-specific history
- All methods are thread-safe with mutex synchronization
- Methods return data in reverse chronological order (newest first)

This fixes the 'undefined method get_recent_backups' error in health endpoints.

app/lib/baktainer/backup_monitor.rb

@@ -230,4 +230,27 @@ class Baktainer::BackupMonitor
       end
     end
   end
+
+  # Get recent backups for health check endpoints
+  def get_recent_backups(limit = 50)
+    @mutex.synchronize do
+      @backup_history.last(limit).reverse
+    end
+  end
+
+  # Get failed backups for health check endpoints  
+  def get_failed_backups(limit = 20)
+    @mutex.synchronize do
+      failed = @backup_history.select { |b| b[:status] == 'failed' }
+      failed.last(limit).reverse
+    end
+  end
+
+  # Get backup history for a specific container
+  def get_container_backup_history(container_name, limit = 20)
+    @mutex.synchronize do
+      container_backups = @backup_history.select { |b| b[:container_name] == container_name }
+      container_backups.last(limit).reverse
+    end
+  end
 end