Make the cache stats count what is cached, fetched and transcoded (closes #56)

Stats read request_cache and output_content, which nothing writes, so
TotalItems and TotalSizeBytes were always 0. They now count
source_content plus variant_content, the size through UsageBytes; a
failed query is still logged at warn. Get counts a miss after the work,
passing the bytes fetched from upstream (0 for a cached source; still
counted when the fetched source then fails), so upstream_fetch_count and
upstream_fetch_bytes move. transform_count is incremented after each
successful image processor call. request_cache and output_content stay
in the schema; dropping them is a separate decision.

Model: opus-5-5

internal/imgcache/service.go

@@ -155,12 +155,14 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 		}
 	}
 
-	// Cache miss - check if we have source content cached
+	// Cache miss - process the cached source or fetch it, then count the
+	// miss with the bytes it fetched from upstream, also when it failed
 	cacheKey := CacheKey(req)
 
-	s.cache.IncrementStats(ctx, false, 0)
+	response, fetchedBytes, err := s.processFromSourceOrFetch(ctx, req, cacheKey)
+
+	s.cache.IncrementStats(ctx, false, fetchedBytes)
 
-	response, err := s.processFromSourceOrFetch(ctx, req, cacheKey)
 	if err != nil {
 		return nil, err
 	}
@@ -268,12 +270,13 @@ func (s *Service) loadCachedSource(contentHash ContentHash) []byte {
 }
 
 // processFromSourceOrFetch processes an image, using cached source content
-// if available.
+// if available. It also returns the number of bytes fetched from upstream,
+// as fetchAndProcess does, or 0 when the cached source was used.
 func (s *Service) processFromSourceOrFetch(
 	ctx context.Context,
 	req *ImageRequest,
 	cacheKey VariantKey,
-) (*ImageResponse, error) {
+) (*ImageResponse, int64, error) {
 	// Check if we have cached source content
 	contentHash, _, err := s.cache.LookupSource(ctx, req)
 	if err != nil {
@@ -292,26 +295,25 @@ func (s *Service) processFromSourceOrFetch(
 
 	// Fetch from upstream if we don't have source data or it's empty
 	if len(sourceData) == 0 {
-		resp, err := s.fetchAndProcess(ctx, req, cacheKey)
-		if err != nil {
-			return nil, err
-		}
-
-		return resp, nil
+		return s.fetchAndProcess(ctx, req, cacheKey)
 	}
 
-	// Process using cached source
+	// Process using cached source; nothing was fetched from upstream
 	fetchBytes = int64(len(sourceData))
 
-	return s.processAndStore(ctx, req, cacheKey, sourceData, fetchBytes)
+	resp, err := s.processAndStore(ctx, req, cacheKey, sourceData, fetchBytes)
+
+	return resp, 0, err
 }
 
 // fetchAndProcess fetches from upstream, processes, and caches the result.
+// It also returns the number of bytes fetched from upstream once the
+// response has been read, including when a later step fails.
 func (s *Service) fetchAndProcess(
 	ctx context.Context,
 	req *ImageRequest,
 	cacheKey VariantKey,
-) (*ImageResponse, error) {
+) (*ImageResponse, int64, error) {
 	// Fetch from upstream
 	sourceURL := req.SourceURL()
 
@@ -330,7 +332,7 @@ func (s *Service) fetchAndProcess(
 			}
 		}
 
-		return nil, fmt.Errorf("upstream fetch failed: %w", err)
+		return nil, 0, fmt.Errorf("upstream fetch failed: %w", err)
 	}
 
 	defer func() { _ = fetchResult.Content.Close() }()
@@ -338,7 +340,7 @@ func (s *Service) fetchAndProcess(
 	// Read and validate the source content
 	sourceData, err := io.ReadAll(fetchResult.Content)
 	if err != nil {
-		return nil, fmt.Errorf("failed to read upstream response: %w", err)
+		return nil, 0, fmt.Errorf("failed to read upstream response: %w", err)
 	}
 
 	// Calculate download bitrate
@@ -368,7 +370,7 @@ func (s *Service) fetchAndProcess(
 	// Validate magic bytes match content type
 	err = magic.ValidateMagicBytes(sourceData, fetchResult.ContentType)
 	if err != nil {
-		return nil, fmt.Errorf("content validation failed: %w", err)
+		return nil, fetchBytes, fmt.Errorf("content validation failed: %w", err)
 	}
 
 	// Store source content
@@ -378,7 +380,9 @@ func (s *Service) fetchAndProcess(
 		// Continue even if caching fails
 	}
 
-	return s.processAndStore(ctx, req, cacheKey, sourceData, fetchBytes)
+	resp, err := s.processAndStore(ctx, req, cacheKey, sourceData, fetchBytes)
+
+	return resp, fetchBytes, err
 }
 
 // processAndStore processes an image and stores the result.
@@ -406,6 +410,8 @@ func (s *Service) processAndStore(
 
 	processDuration := time.Since(processStart)
 
+	s.cache.IncrementTransformCount(ctx)
+
 	// Read processed content
 	processedData, err := io.ReadAll(processResult.Content)
 	_ = processResult.Content.Close()