Refactor to serve all responses from cached files on disk

- StoreOutput now returns output hash for immediate retrieval
- Cache misses now serve from disk file after storing (same as hits)
- Log served_bytes from actual io.Copy result (avoids stat calls)
- Remove ContentLength field usage for cache hits (stream from file)
- Fix tests to properly check all return values

internal/imgcache/cache.go

@@ -231,18 +231,18 @@ func (c *Cache) StoreSource(
 	return contentHash, nil
 }
 
-// StoreOutput stores processed output content.
+// StoreOutput stores processed output content and returns the output hash.
 func (c *Cache) StoreOutput(
 	ctx context.Context,
 	req *ImageRequest,
 	sourceMetadataID int64,
 	content io.Reader,
 	contentType string,
-) error {
+) (string, error) {
 	// Store content
 	outputHash, size, err := c.dstContent.Store(content)
 	if err != nil {
-		return fmt.Errorf("failed to store output content: %w", err)
+		return "", fmt.Errorf("failed to store output content: %w", err)
 	}
 
 	cacheKey := CacheKey(req)
@@ -254,7 +254,7 @@ func (c *Cache) StoreOutput(
 		ON CONFLICT(content_hash) DO NOTHING
 	`, outputHash, contentType, size)
 	if err != nil {
-		return fmt.Errorf("failed to insert output content: %w", err)
+		return "", fmt.Errorf("failed to insert output content: %w", err)
 	}
 
 	_, err = c.db.ExecContext(ctx, `
@@ -266,7 +266,7 @@ func (c *Cache) StoreOutput(
 			access_count = request_cache.access_count + 1
 	`, cacheKey, sourceMetadataID, outputHash, req.Size.Width, req.Size.Height, req.Format, req.Quality, req.FitMode)
 	if err != nil {
-		return fmt.Errorf("failed to insert request cache: %w", err)
+		return "", fmt.Errorf("failed to insert request cache: %w", err)
 	}
 
 	// Update hot cache
@@ -276,7 +276,7 @@ func (c *Cache) StoreOutput(
 		c.hotCacheMu.Unlock()
 	}
 
-	return nil
+	return outputHash, nil
 }
 
 // StoreNegative stores a negative cache entry for a failed fetch.