refactor: extract httpfetcher package from imgcache

Move HTTPFetcher, Config (was FetcherConfig), SSRF-safe dialer, rate
limiting, content-type validation, and related error vars from
internal/imgcache/fetcher.go into new internal/httpfetcher/ package.

The Fetcher interface and FetchResult type also move to httpfetcher
to avoid circular imports (imgcache imports httpfetcher, not the other
way around).

Renames to avoid stuttering:
  NewHTTPFetcher -> httpfetcher.New
  FetcherConfig  -> httpfetcher.Config
  NewMockFetcher -> httpfetcher.NewMock

The ServiceConfig.FetcherConfig field is retained (it describes what
kind of config it holds, not a stutter).

Pure refactor - no behavior changes. Unit tests for the httpfetcher
package are included.

refs #39

internal/imgcache/service.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/dustin/go-humanize"
 	"sneak.berlin/go/pixa/internal/allowlist"
+	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imageprocessor"
 	"sneak.berlin/go/pixa/internal/magic"
 )
@@ -19,7 +20,7 @@ import (
 // Service implements the ImageCache interface, orchestrating cache, fetcher, and processor.
 type Service struct {
 	cache           *Cache
-	fetcher         Fetcher
+	fetcher         httpfetcher.Fetcher
 	processor       *imageprocessor.ImageProcessor
 	signer          *Signer
 	allowlist       *allowlist.HostAllowList
@@ -33,9 +34,9 @@ type ServiceConfig struct {
 	// Cache is the cache instance
 	Cache *Cache
 	// FetcherConfig configures the upstream fetcher (ignored if Fetcher is set)
-	FetcherConfig *FetcherConfig
+	FetcherConfig *httpfetcher.Config
 	// Fetcher is an optional custom fetcher (for testing)
-	Fetcher Fetcher
+	Fetcher httpfetcher.Fetcher
 	// SigningKey is the HMAC signing key (empty disables signing)
 	SigningKey string
 	// Whitelist is the list of hosts that don't require signatures
@@ -57,15 +58,15 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 	// Resolve fetcher config for defaults
 	fetcherCfg := cfg.FetcherConfig
 	if fetcherCfg == nil {
-		fetcherCfg = DefaultFetcherConfig()
+		fetcherCfg = httpfetcher.DefaultConfig()
 	}
 
 	// Use custom fetcher if provided, otherwise create HTTP fetcher
-	var fetcher Fetcher
+	var fetcher httpfetcher.Fetcher
 	if cfg.Fetcher != nil {
 		fetcher = cfg.Fetcher
 	} else {
-		fetcher = NewHTTPFetcher(fetcherCfg)
+		fetcher = httpfetcher.New(fetcherCfg)
 	}
 
 	signer := NewSigner(cfg.SigningKey)
@@ -113,7 +114,7 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 			"path", req.SourcePath,
 		)
 
-		return nil, fmt.Errorf("%w: %w", ErrUpstreamError, ErrNegativeCached)
+		return nil, fmt.Errorf("%w: %w", httpfetcher.ErrUpstreamError, ErrNegativeCached)
 	}
 
 	// Check variant cache first (disk only, no DB)
@@ -418,13 +419,13 @@ const (
 
 // isNegativeCacheable returns true if the error should be cached.
 func isNegativeCacheable(err error) bool {
-	return errors.Is(err, ErrUpstreamError)
+	return errors.Is(err, httpfetcher.ErrUpstreamError)
 }
 
 // extractStatusCode extracts HTTP status code from error message.
 func extractStatusCode(err error) int {
 	// Default to 502 Bad Gateway for upstream errors
-	if errors.Is(err, ErrUpstreamError) {
+	if errors.Is(err, httpfetcher.ErrUpstreamError) {
 		return httpStatusBadGateway
 	}