fix(backend): invalidate thumbnail cache on replace and permanent delete

Replacing a file's content left the old {id}_thumb.jpg / {id}_preview.jpg
in the cache, and the cache-hit fast path kept serving the stale image
forever; permanent deletion left those files orphaned. FileStorage gains
InvalidateCache, which Replace and PermanentDelete now call.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/port/storage.go

@@ -21,6 +21,10 @@ type FileStorage interface {
 	// Delete removes the file content from storage.
 	Delete(ctx context.Context, id uuid.UUID) error
 
+	// InvalidateCache removes any cached thumbnail/preview for the file so they
+	// are regenerated from the current content on next request.
+	InvalidateCache(ctx context.Context, id uuid.UUID) error
+
 	// Thumbnail opens the pre-generated thumbnail (JPEG). Returns ErrNotFound
 	// if the thumbnail has not been generated yet.
 	Thumbnail(ctx context.Context, id uuid.UUID) (io.ReadCloser, error)