Add mock fetcher and service tests for imgcache

Introduces Fetcher interface, mock implementation for testing,
and ApplyMigrations helper for test database setup.

internal/imgcache/service.go

@@ -14,7 +14,7 @@ import (
 // Service implements the ImageCache interface, orchestrating cache, fetcher, and processor.
 type Service struct {
 	cache     *Cache
-	fetcher   *HTTPFetcher
+	fetcher   Fetcher
 	processor Processor
 	signer    *Signer
 	whitelist *HostWhitelist
@@ -25,8 +25,10 @@ type Service struct {
 type ServiceConfig struct {
 	// Cache is the cache instance
 	Cache *Cache
-	// FetcherConfig configures the upstream fetcher
+	// FetcherConfig configures the upstream fetcher (ignored if Fetcher is set)
 	FetcherConfig *FetcherConfig
+	// Fetcher is an optional custom fetcher (for testing)
+	Fetcher Fetcher
 	// SigningKey is the HMAC signing key (empty disables signing)
 	SigningKey string
 	// Whitelist is the list of hosts that don't require signatures
@@ -41,9 +43,16 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 		return nil, errors.New("cache is required")
 	}
 
-	fetcherCfg := cfg.FetcherConfig
-	if fetcherCfg == nil {
-		fetcherCfg = DefaultFetcherConfig()
+	// Use custom fetcher if provided, otherwise create HTTP fetcher
+	var fetcher Fetcher
+	if cfg.Fetcher != nil {
+		fetcher = cfg.Fetcher
+	} else {
+		fetcherCfg := cfg.FetcherConfig
+		if fetcherCfg == nil {
+			fetcherCfg = DefaultFetcherConfig()
+		}
+		fetcher = NewHTTPFetcher(fetcherCfg)
 	}
 
 	var signer *Signer
@@ -58,7 +67,7 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 
 	return &Service{
 		cache:     cfg.Cache,
-		fetcher:   NewHTTPFetcher(fetcherCfg),
+		fetcher:   fetcher,
 		processor: NewImageProcessor(),
 		signer:    signer,
 		whitelist: NewHostWhitelist(cfg.Whitelist),