refactor: extract magic byte detection into internal/magic package

Move MIMEType, DetectFormat, ValidateMagicBytes, PeekAndValidate,
IsSupportedMIMEType, MIMEToImageFormat, and ImageFormatToMIME from
internal/imgcache into a new internal/magic package.

The magic package defines its own ImageFormat type and constants to
avoid circular imports (imgcache imports magic for validation,
magic cannot import imgcache).

Update import sites in imgcache/service.go and service_test.go.

Part of issue #39.

internal/allowlist/allowlist.go

@@ -1,25 +1,26 @@
-package imgcache
+// Package allowlist provides host-based URL allow-listing for the image proxy.
+package allowlist
 
 import (
 	"net/url"
 	"strings"
 )
 
-// HostWhitelist implements the Whitelist interface for checking allowed source hosts.
-type HostWhitelist struct {
+// HostAllowList checks whether source hosts are permitted.
+type HostAllowList struct {
 	// exactHosts contains hosts that must match exactly (e.g., "cdn.example.com")
 	exactHosts map[string]struct{}
 	// suffixHosts contains domain suffixes to match (e.g., ".example.com" matches "cdn.example.com")
 	suffixHosts []string
 }
 
-// NewHostWhitelist creates a whitelist from a list of host patterns.
+// New creates a HostAllowList from a list of host patterns.
 // Patterns starting with "." are treated as suffix matches.
 // Examples:
 //   - "cdn.example.com" - exact match only
 //   - ".example.com" - matches cdn.example.com, images.example.com, etc.
-func NewHostWhitelist(patterns []string) *HostWhitelist {
-	w := &HostWhitelist{
+func New(patterns []string) *HostAllowList {
+	w := &HostAllowList{
 		exactHosts:  make(map[string]struct{}),
 		suffixHosts: make([]string, 0),
 	}
@@ -40,8 +41,8 @@ func NewHostWhitelist(patterns []string) *HostWhitelist {
 	return w
 }
 
-// IsWhitelisted checks if a URL's host is in the whitelist.
-func (w *HostWhitelist) IsWhitelisted(u *url.URL) bool {
+// IsAllowed checks if a URL's host is in the allow list.
+func (w *HostAllowList) IsAllowed(u *url.URL) bool {
 	if u == nil {
 		return false
 	}
@@ -71,12 +72,12 @@ func (w *HostWhitelist) IsWhitelisted(u *url.URL) bool {
 	return false
 }
 
-// IsEmpty returns true if the whitelist has no entries.
-func (w *HostWhitelist) IsEmpty() bool {
+// IsEmpty returns true if the allow list has no entries.
+func (w *HostAllowList) IsEmpty() bool {
 	return len(w.exactHosts) == 0 && len(w.suffixHosts) == 0
 }
 
-// Count returns the total number of whitelist entries.
-func (w *HostWhitelist) Count() int {
+// Count returns the total number of allow list entries.
+func (w *HostAllowList) Count() int {
 	return len(w.exactHosts) + len(w.suffixHosts)
 }

internal/allowlist/allowlist_test.go

@@ -1,11 +1,13 @@
-package imgcache
+package allowlist_test
 
 import (
 	"net/url"
 	"testing"
+
+	"sneak.berlin/go/pixa/internal/allowlist"
 )
 
-func TestHostWhitelist_IsWhitelisted(t *testing.T) {
+func TestHostAllowList_IsAllowed(t *testing.T) {
 	tests := []struct {
 		name     string
 		patterns []string
@@ -67,7 +69,7 @@ func TestHostWhitelist_IsWhitelisted(t *testing.T) {
 			want:     true,
 		},
 		{
-			name:     "empty whitelist",
+			name:     "empty allow list",
 			patterns: []string{},
 			testURL:  "https://cdn.example.com/image.jpg",
 			want:     false,
@@ -94,7 +96,7 @@ func TestHostWhitelist_IsWhitelisted(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			w := NewHostWhitelist(tt.patterns)
+			w := allowlist.New(tt.patterns)
 
 			var u *url.URL
 			if tt.testURL != "" {
@@ -105,15 +107,15 @@ func TestHostWhitelist_IsWhitelisted(t *testing.T) {
 				}
 			}
 
-			got := w.IsWhitelisted(u)
+			got := w.IsAllowed(u)
 			if got != tt.want {
-				t.Errorf("IsWhitelisted() = %v, want %v", got, tt.want)
+				t.Errorf("IsAllowed() = %v, want %v", got, tt.want)
 			}
 		})
 	}
 }
 
-func TestHostWhitelist_IsEmpty(t *testing.T) {
+func TestHostAllowList_IsEmpty(t *testing.T) {
 	tests := []struct {
 		name     string
 		patterns []string
@@ -143,7 +145,7 @@ func TestHostWhitelist_IsEmpty(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			w := NewHostWhitelist(tt.patterns)
+			w := allowlist.New(tt.patterns)
 			if got := w.IsEmpty(); got != tt.want {
 				t.Errorf("IsEmpty() = %v, want %v", got, tt.want)
 			}
@@ -151,7 +153,7 @@ func TestHostWhitelist_IsEmpty(t *testing.T) {
 	}
 }
 
-func TestHostWhitelist_Count(t *testing.T) {
+func TestHostAllowList_Count(t *testing.T) {
 	tests := []struct {
 		name     string
 		patterns []string
@@ -181,7 +183,7 @@ func TestHostWhitelist_Count(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			w := NewHostWhitelist(tt.patterns)
+			w := allowlist.New(tt.patterns)
 			if got := w.Count(); got != tt.want {
 				t.Errorf("Count() = %v, want %v", got, tt.want)
 			}

internal/imgcache/service.go

@@ -11,7 +11,9 @@ import (
 	"time"
 
 	"github.com/dustin/go-humanize"
+	"sneak.berlin/go/pixa/internal/allowlist"
 	"sneak.berlin/go/pixa/internal/imageprocessor"
+	"sneak.berlin/go/pixa/internal/magic"
 )
 
 // Service implements the ImageCache interface, orchestrating cache, fetcher, and processor.
@@ -20,7 +22,7 @@ type Service struct {
 	fetcher         Fetcher
 	processor       *imageprocessor.ImageProcessor
 	signer          *Signer
-	whitelist       *HostWhitelist
+	allowlist       *allowlist.HostAllowList
 	log             *slog.Logger
 	allowHTTP       bool
 	maxResponseSize int64
@@ -85,7 +87,7 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 		fetcher:         fetcher,
 		processor:       imageprocessor.New(imageprocessor.Params{MaxInputBytes: maxResponseSize}),
 		signer:          signer,
-		whitelist:       NewHostWhitelist(cfg.Whitelist),
+		allowlist:       allowlist.New(cfg.Whitelist),
 		log:             log,
 		allowHTTP:       allowHTTP,
 		maxResponseSize: maxResponseSize,
@@ -276,7 +278,7 @@ func (s *Service) fetchAndProcess(
 	)
 
 	// Validate magic bytes match content type
-	if err := ValidateMagicBytes(sourceData, fetchResult.ContentType); err != nil {
+	if err := magic.ValidateMagicBytes(sourceData, fetchResult.ContentType); err != nil {
 		return nil, fmt.Errorf("content validation failed: %w", err)
 	}
 
@@ -381,7 +383,7 @@ func (s *Service) Stats(ctx context.Context) (*CacheStats, error) {
 
 // ValidateRequest validates the request signature if required.
 func (s *Service) ValidateRequest(req *ImageRequest) error {
-	// Check if host is whitelisted (no signature required)
+	// Check if host is allowed (no signature required)
 	sourceURL := req.SourceURL()
 
 	parsedURL, err := url.Parse(sourceURL)
@@ -389,11 +391,11 @@ func (s *Service) ValidateRequest(req *ImageRequest) error {
 		return fmt.Errorf("invalid source URL: %w", err)
 	}
 
-	if s.whitelist.IsWhitelisted(parsedURL) {
+	if s.allowlist.IsAllowed(parsedURL) {
 		return nil
 	}
 
-	// Signature required for non-whitelisted hosts
+	// Signature required for non-allowed hosts
 	return s.signer.Verify(req)
 }
 

internal/imgcache/service_test.go

@@ -5,6 +5,8 @@ import (
 	"io"
 	"testing"
 	"time"
+
+	"sneak.berlin/go/pixa/internal/magic"
 )
 
 func TestService_Get_WhitelistedHost(t *testing.T) {
@@ -315,17 +317,17 @@ func TestService_Get_FormatConversion(t *testing.T) {
 				t.Fatalf("failed to read response: %v", err)
 			}
 
-			detectedMIME, err := DetectFormat(data)
+			detectedMIME, err := magic.DetectFormat(data)
 			if err != nil {
 				t.Fatalf("failed to detect format: %v", err)
 			}
 
-			expectedFormat, ok := MIMEToImageFormat(tt.wantMIME)
+			expectedFormat, ok := magic.MIMEToImageFormat(tt.wantMIME)
 			if !ok {
 				t.Fatalf("unknown format for MIME type: %s", tt.wantMIME)
 			}
 
-			detectedFormat, ok := MIMEToImageFormat(string(detectedMIME))
+			detectedFormat, ok := magic.MIMEToImageFormat(string(detectedMIME))
 			if !ok {
 				t.Fatalf("unknown format for detected MIME type: %s", detectedMIME)
 			}

internal/magic/magic.go

@@ -1,4 +1,6 @@
-package imgcache
+// Package magic detects image formats from magic bytes and validates
+// content against declared MIME types.
+package magic
 
 import (
 	"bytes"
@@ -27,6 +29,20 @@ const (
 	MIMETypeSVG  = MIMEType("image/svg+xml")
 )
 
+// ImageFormat represents supported output image formats.
+// This mirrors the type in imgcache to avoid circular imports.
+type ImageFormat string
+
+// Supported image output formats.
+const (
+	FormatOriginal ImageFormat = "orig"
+	FormatJPEG     ImageFormat = "jpeg"
+	FormatPNG      ImageFormat = "png"
+	FormatWebP     ImageFormat = "webp"
+	FormatAVIF     ImageFormat = "avif"
+	FormatGIF      ImageFormat = "gif"
+)
+
 // MinMagicBytes is the minimum number of bytes needed to detect format.
 const MinMagicBytes = 12
 
@@ -189,7 +205,7 @@ func PeekAndValidate(r io.Reader, declaredType string) (io.Reader, error) {
 	return io.MultiReader(bytes.NewReader(buf), r), nil
 }
 
-// MIMEToImageFormat converts a MIME type to our ImageFormat type.
+// MIMEToImageFormat converts a MIME type to an ImageFormat.
 func MIMEToImageFormat(mimeType string) (ImageFormat, bool) {
 	normalized := normalizeMIMEType(mimeType)
 	switch MIMEType(normalized) {
@@ -208,7 +224,7 @@ func MIMEToImageFormat(mimeType string) (ImageFormat, bool) {
 	}
 }
 
-// ImageFormatToMIME converts our ImageFormat to a MIME type string.
+// ImageFormatToMIME converts an ImageFormat to a MIME type string.
 func ImageFormatToMIME(format ImageFormat) string {
 	switch format {
 	case FormatJPEG:

internal/magic/magic_test.go

@@ -1,4 +1,4 @@
-package imgcache
+package magic
 
 import (
 	"bytes"