feat: show disk usage of mover source with configurable warning threshold

GET /api/storage/disk-usage returns source path stats (total/used/free bytes, %).
Polls every 30s while Storage page is open.

Backend:
- migration 0009: adds mover_warning_threshold (1-99, default 80) to storage_config
- StatPath moved to internal/storage package for testability
- handleStorageDiskUsage returns {source: PathStat}

Frontend:
- DiskUsageBar component in Mergerfs Mover card: usage bar with color
  green < threshold, amber >= threshold, red >= 95%
- Warning message when threshold reached or exceeded
- New input in config form: Umbral de aviso (%)
- storageDiskUsage() API method

Tests: StatPath unit tests (empty/nonexistent/valid paths)

Version: 0.7.3

internal/db/models.go

@@ -95,7 +95,8 @@ type StorageConfig struct {
 	MoverCleanMacOS   bool   `json:"mover_clean_macos"`
 	MoverRemoveSource bool   `json:"mover_remove_source"`
 	MoverInplace      bool   `json:"mover_inplace"`
-	MoverRsyncOptions string `json:"mover_rsync_options"`
+	MoverRsyncOptions      string `json:"mover_rsync_options"`
+	MoverWarningThreshold  int    `json:"mover_warning_threshold"`
 	SnapraidContent   string `json:"snapraid_content"`
 	SnapraidDataDirs  string `json:"snapraid_data_dirs"`
 	SnapraidParityDir string `json:"snapraid_parity_dir"`