refactor: extract httpfetcher package from imgcache (#43)

Extracts the HTTP fetcher concern out of `internal/imgcache/` into its own `internal/httpfetcher/` package, per the plan in [issue #39](#39).

This is one of four planned extractions; only the fetcher is moved here so the diff stays reviewable. The remaining three (signature, magic, urlparser) will land in separate PRs.

## What moved

From `internal/imgcache/fetcher.go` and `internal/imgcache/mock_fetcher.go` into `internal/httpfetcher/`:

- `HTTPFetcher` type, its `New` constructor, and `Config` (formerly `FetcherConfig`) with `DefaultConfig`
- SSRF-safe dialer, `validateURL`, `isPrivateIP`, `isLocalhost`, `extractHost`
- Per-host connection semaphore (rate limiting) and `limitedReader`
- Content-type validation (`isAllowedContentType`, `detectContentTypeFromPath`)
- All related error values (`ErrSSRFBlocked`, `ErrUpstreamError`, `ErrUpstreamTimeout`, `ErrPayloadTooLarge`, `ErrDisallowedContentType`)
- All related constants (`DefaultFetchTimeout`, `DefaultMaxPayloadBytes`, `DefaultMaxConnectionsPerHost`, etc.)
- The `Fetcher` interface and `FetchResult` type (moved here to keep the import edge one-way: `imgcache` depends on `httpfetcher`, never the reverse)
- `MockFetcher` test helper

## Renames (no stuttering)

- `NewHTTPFetcher` → `httpfetcher.New`
- `FetcherConfig` → `httpfetcher.Config`
- `DefaultFetcherConfig` → `httpfetcher.DefaultConfig`
- `NewMockFetcher` → `httpfetcher.NewMock`

The `ServiceConfig.FetcherConfig` field name is retained — it describes what kind of config the field holds (not a stutter).

## Behavior

Pure refactor. No behavior changes. All existing tests pass; unit tests for the new package are included.

`docker build .` passes (fmt-check, lint, test, build).

refs #39

Co-authored-by: clawbot <clawbot@eeqj.de>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #43
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

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)