feat(admin): implement provider-specific storage configuration pages

Refactor the admin storage backend creation and editing flows to use
provider-specific pages (e.g., `/admin/storage/new/sftp`) instead of a
single generic form. This ensures only relevant fields are rendered for
each storage provider (such as SFTP, S3, or WebDAV).

Additionally:
- Prevent mutation of the storage provider type during backend edits.
- Add comprehensive unit tests for provider-specific rendering, edit
  validation, and CSRF/admin route protection.

backend/libs/jobs/thumbnails.go

@@ -20,7 +20,7 @@ import (
 	"warpbox.dev/backend/libs/services"
 )
 
-type thumbnailJobResult struct {
+type ThumbnailJobResult struct {
 	Scanned   int
 	Generated int
 	Failed    int
@@ -63,13 +63,17 @@ func newThumbnailsJob(cfg config.Config, logger *slog.Logger, uploadService *ser
 	}
 }
 
-func generateMissingThumbnails(uploadService *services.UploadService, logger *slog.Logger) (thumbnailJobResult, error) {
+func RunThumbnailsNow(uploadService *services.UploadService, logger *slog.Logger) (ThumbnailJobResult, error) {
+	return generateMissingThumbnails(uploadService, logger)
+}
+
+func generateMissingThumbnails(uploadService *services.UploadService, logger *slog.Logger) (ThumbnailJobResult, error) {
 	boxes, err := uploadService.ListBoxes(0)
 	if err != nil {
-		return thumbnailJobResult{}, err
+		return ThumbnailJobResult{}, err
 	}
 
-	var result thumbnailJobResult
+	var result ThumbnailJobResult
 	now := time.Now().UTC()
 	for _, box := range boxes {
 		if !box.ExpiresAt.After(now) {
@@ -88,8 +92,8 @@ func generateMissingThumbnails(uploadService *services.UploadService, logger *sl
 	return result, nil
 }
 
-func generateMissingThumbnailsForBox(uploadService *services.UploadService, logger *slog.Logger, box services.Box) (thumbnailJobResult, error) {
-	var result thumbnailJobResult
+func generateMissingThumbnailsForBox(uploadService *services.UploadService, logger *slog.Logger, box services.Box) (ThumbnailJobResult, error) {
+	var result ThumbnailJobResult
 	if !box.ExpiresAt.After(time.Now().UTC()) {
 		return result, nil
 	}