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/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)
 			}