Fix hot cache to include ContentType and SizeBytes

Hot cache entries now store all data needed to serve a cache hit
without any database access:
- OutputHash (for file lookup)
- ContentType (for Content-Type header)
- SizeBytes (for Content-Length header)

Previously hot cache only stored OutputHash, causing empty
Content-Type headers on cached WebP responses.

internal/imgcache/cache.go

@@ -30,6 +30,13 @@ type CacheConfig struct {
 	HotCacheEnabled bool
 }
 
+// hotCacheEntry stores all data needed to serve a cache hit without DB access.
+type hotCacheEntry struct {
+	OutputHash  string
+	ContentType string
+	SizeBytes   int64
+}
+
 // Cache implements the caching layer for the image proxy.
 type Cache struct {
 	db              *sql.DB
@@ -37,7 +44,7 @@ type Cache struct {
 	dstContent      *ContentStorage
 	srcMetadata     *MetadataStorage
 	config          CacheConfig
-	hotCache        map[string]string // cache_key -> output_hash
+	hotCache        map[string]hotCacheEntry // cache_key -> entry
 	hotCacheMu      sync.RWMutex
 	hotCacheEnabled bool
 }
@@ -69,7 +76,7 @@ func NewCache(db *sql.DB, config CacheConfig) (*Cache, error) {
 	}
 
 	if config.HotCacheEnabled && config.HotCacheSize > 0 {
-		c.hotCache = make(map[string]string, config.HotCacheSize)
+		c.hotCache = make(map[string]hotCacheEntry, config.HotCacheSize)
 	}
 
 	return c, nil
@@ -80,6 +87,7 @@ type LookupResult struct {
 	Hit         bool
 	OutputHash  string
 	ContentType string
+	SizeBytes   int64
 	CacheStatus CacheStatus
 }
 
@@ -90,13 +98,15 @@ func (c *Cache) Lookup(ctx context.Context, req *ImageRequest) (*LookupResult, e
 	// Check hot cache first
 	if c.hotCacheEnabled {
 		c.hotCacheMu.RLock()
-		outputHash, ok := c.hotCache[cacheKey]
+		entry, ok := c.hotCache[cacheKey]
 		c.hotCacheMu.RUnlock()
 
-		if ok && c.dstContent.Exists(outputHash) {
+		if ok && c.dstContent.Exists(entry.OutputHash) {
 			return &LookupResult{
 				Hit:         true,
-				OutputHash:  outputHash,
+				OutputHash:  entry.OutputHash,
+				ContentType: entry.ContentType,
+				SizeBytes:   entry.SizeBytes,
 				CacheStatus: CacheHit,
 			}, nil
 		}
@@ -114,14 +124,15 @@ func (c *Cache) Lookup(ctx context.Context, req *ImageRequest) (*LookupResult, e
 
 	// Check database
 	var outputHash, contentType string
+	var sizeBytes int64
 	var fetchedAt time.Time
 
 	err = c.db.QueryRowContext(ctx, `
-		SELECT rc.output_hash, oc.content_type, rc.fetched_at
+		SELECT rc.output_hash, oc.content_type, oc.size_bytes, rc.fetched_at
 		FROM request_cache rc
 		JOIN output_content oc ON rc.output_hash = oc.content_hash
 		WHERE rc.cache_key = ?
-	`, cacheKey).Scan(&outputHash, &contentType, &fetchedAt)
+	`, cacheKey).Scan(&outputHash, &contentType, &sizeBytes, &fetchedAt)
 
 	if errors.Is(err, sql.ErrNoRows) {
 		return &LookupResult{Hit: false, CacheStatus: CacheMiss}, nil
@@ -144,7 +155,11 @@ func (c *Cache) Lookup(ctx context.Context, req *ImageRequest) (*LookupResult, e
 	// Update hot cache
 	if c.hotCacheEnabled {
 		c.hotCacheMu.Lock()
-		c.hotCache[cacheKey] = outputHash
+		c.hotCache[cacheKey] = hotCacheEntry{
+			OutputHash:  outputHash,
+			ContentType: contentType,
+			SizeBytes:   sizeBytes,
+		}
 		c.hotCacheMu.Unlock()
 	}
 
@@ -159,6 +174,7 @@ func (c *Cache) Lookup(ctx context.Context, req *ImageRequest) (*LookupResult, e
 		Hit:         true,
 		OutputHash:  outputHash,
 		ContentType: contentType,
+		SizeBytes:   sizeBytes,
 		CacheStatus: CacheHit,
 	}, nil
 }
@@ -277,7 +293,11 @@ func (c *Cache) StoreOutput(
 	// Update hot cache
 	if c.hotCacheEnabled {
 		c.hotCacheMu.Lock()
-		c.hotCache[cacheKey] = outputHash
+		c.hotCache[cacheKey] = hotCacheEntry{
+			OutputHash:  outputHash,
+			ContentType: contentType,
+			SizeBytes:   size,
+		}
 		c.hotCacheMu.Unlock()
 	}