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/handlers/imageenc.go

@@ -11,6 +11,7 @@ import (
 	"github.com/go-chi/chi/v5"
 
 	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imgcache"
 )
 
@@ -100,11 +101,11 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 // handleImageError converts image service errors to HTTP responses.
 func (s *Handlers) handleImageError(w http.ResponseWriter, err error) {
 	switch {
-	case errors.Is(err, imgcache.ErrSSRFBlocked):
+	case errors.Is(err, httpfetcher.ErrSSRFBlocked):
 		s.respondError(w, "forbidden", http.StatusForbidden)
-	case errors.Is(err, imgcache.ErrUpstreamError):
+	case errors.Is(err, httpfetcher.ErrUpstreamError):
 		s.respondError(w, "upstream error", http.StatusBadGateway)
-	case errors.Is(err, imgcache.ErrUpstreamTimeout):
+	case errors.Is(err, httpfetcher.ErrUpstreamTimeout):
 		s.respondError(w, "upstream timeout", http.StatusGatewayTimeout)
 	default:
 		s.log.Error("image request failed", "error", err)