Check cached file sizes at startup, not per request (closes #252)

Lookup and LookupSource are as on next again: requests compare no
sizes and a hit reads no extra row. The reconciliation pass, at startup
and every five minutes, compares each cached source and variant file
with the size its row records and removes one that is empty or of
another size, with its rows and a warning naming the file; an empty
variant file without a row is removed rather than adopted. Source,
variant and .meta files all go through writeFileSynced (temp file,
sync, rename, directory sync), the .meta file before the variant, and
a failed directory sync removes the renamed file.

#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
+	// Check variant cache first (disk only, no DB)
 	result, err := s.cache.Lookup(ctx, req)
 	if err != nil {
 		s.log.Warn("cache lookup failed", "error", err)