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/imgcache.go

@@ -169,36 +169,6 @@ type Whitelist interface {
 	IsWhitelisted(u *url.URL) bool
 }
 
-// Fetcher fetches images from upstream origins
-type Fetcher interface {
-	// Fetch retrieves an image from the origin
-	Fetch(ctx context.Context, url string) (*FetchResult, error)
-}
-
-// FetchResult contains the result of fetching from upstream
-type FetchResult struct {
-	// Content is the raw image data
-	Content io.ReadCloser
-	// ContentLength is the size in bytes (-1 if unknown)
-	ContentLength int64
-	// ContentType is the MIME type from upstream
-	ContentType string
-	// Headers contains all response headers from upstream
-	Headers map[string][]string
-	// StatusCode is the HTTP status code from upstream
-	StatusCode int
-	// FetchDurationMs is how long the fetch took in milliseconds
-	FetchDurationMs int64
-	// RemoteAddr is the IP:port of the upstream server
-	RemoteAddr string
-	// HTTPVersion is the protocol version (e.g., "1.1", "2.0")
-	HTTPVersion string
-	// TLSVersion is the TLS protocol version (e.g., "TLS 1.3")
-	TLSVersion string
-	// TLSCipherSuite is the negotiated cipher suite name
-	TLSCipherSuite string
-}
-
 // Storage handles persistent storage of cached content
 type Storage interface {
 	// Store saves content and returns its hash