Add type-safe hash types for cache storage

Define ContentHash, VariantKey, and PathHash types to replace
raw strings, providing compile-time type safety for storage
operations. Update storage layer to use typed parameters,
refactor cache to use variant storage keyed by VariantKey,
and implement source content reuse on cache misses.

internal/handlers/handlers.go

@@ -61,11 +61,9 @@ func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
 func (s *Handlers) initImageService() error {
 	// Create the cache
 	cache, err := imgcache.NewCache(s.db.DB(), imgcache.CacheConfig{
-		StateDir:        s.config.StateDir,
-		CacheTTL:        imgcache.DefaultCacheTTL,
-		NegativeTTL:     imgcache.DefaultNegativeTTL,
-		HotCacheSize:    imgcache.DefaultHotCacheSize,
-		HotCacheEnabled: true,
+		StateDir:    s.config.StateDir,
+		CacheTTL:    imgcache.DefaultCacheTTL,
+		NegativeTTL: imgcache.DefaultNegativeTTL,
 	})
 	if err != nil {
 		return err