backend: add force rescan parameter to scanner

- Add Force bool field to ScanLibraryRequest in handlers
- Pass force param through job params to worker
- Add forceRescan field and SetForce method to MediaScanner
- Modify processMediaFile to delete and re-create existing items when force=true
- Default behavior unchanged (force=false maintains skip-if-exists)

internal/handlers/scanner.go

@@ -89,6 +89,7 @@ func SetupRoutes(g *echo.Group, db *database.Queries, connManager *wsync.Connect
 type ScanLibraryRequest struct {
 	FolderPaths []string `json:"folder_paths,omitempty"`
 	LibraryID   string   `json:"library_id,omitempty"`
+	Force       bool     `json:"force,omitempty"`
 }
 
 // ScanLibrary handles POST /api/scanner/scan (now runs in background)
@@ -155,6 +156,7 @@ func (h *Handler) ScanLibrary(c echo.Context) error {
 			"folders":    folderPaths,
 			"admin_id":   userUUID.String(),
 			"db":         h.db,
+			"force":      req.Force,
 		},
 		Status:  services.JobStatusPending,
 		Context: h.ctx,