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/handlers_test.go

@@ -18,6 +18,7 @@ import (
 
 	"github.com/go-chi/chi/v5"
 	"sneak.berlin/go/pixa/internal/database"
+	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imgcache"
 )
 
@@ -116,16 +117,16 @@ func newMockFetcher(fs fs.FS) *mockFetcher {
 	return &mockFetcher{fs: fs}
 }
 
-func (f *mockFetcher) Fetch(ctx context.Context, url string) (*imgcache.FetchResult, error) {
+func (f *mockFetcher) Fetch(ctx context.Context, url string) (*httpfetcher.FetchResult, error) {
 	// Remove https:// prefix
 	path := url[8:] // Remove "https://"
 
 	data, err := fs.ReadFile(f.fs, path)
 	if err != nil {
-		return nil, imgcache.ErrUpstreamError
+		return nil, httpfetcher.ErrUpstreamError
 	}
 
-	return &imgcache.FetchResult{
+	return &httpfetcher.FetchResult{
 		Content:       io.NopCloser(bytes.NewReader(data)),
 		ContentLength: int64(len(data)),
 		ContentType:   "image/jpeg",