feat(CUB-39): create background job for filament usage sync

backend/Domain/Interfaces/IFilamentUsageSyncService.cs

@@ -0,0 +1,19 @@
+namespace Extrudex.Domain.Interfaces;
+
+/// <summary>
+/// Service interface for syncing filament usage data from printers
+/// into the Extrudex database. Handles querying Moonraker printers,
+/// computing derived usage metrics, and persisting updates to spools
+/// and print job records.
+/// </summary>
+public interface IFilamentUsageSyncService
+{
+    /// <summary>
+    /// Performs a single sync cycle: queries all active Moonraker printers,
+    /// fetches their current filament usage data, and persists updates to
+    /// the database.
+    /// </summary>
+    /// <param name="cancellationToken">Cancellation token for graceful shutdown.</param>
+    /// <returns>The number of printers successfully synced.</returns>
+    Task<int> SyncAllAsync(CancellationToken cancellationToken = default);
+}