Sync cached files and refuse ones left empty or partial (closes #252)

Source and variant files are synced to disk before the rename into
place, and their directory after. Lookup and LookupSource compare the
file's size with the size its row records: a file that is empty or of
another size is removed with its rows and a warning naming it, and the
request converts the image or fetches the source again. A variant whose
row cannot be read is judged by its file alone. Storing a source over a
file of another size replaces it. A source that fails to decode is not
removed for that alone.

#252

Model: opus-5-5

internal/imgcache/service.go

@@ -148,7 +148,7 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 		return nil, fmt.Errorf("%w: %w", httpfetcher.ErrUpstreamError, ErrNegativeCached)
 	}
 
-	// Check variant cache first (disk only, no DB)
+	// Check variant cache first
 	result, err := s.cache.Lookup(ctx, req)
 	if err != nil {
 		s.log.Warn("cache lookup failed", "error", err)