refactor: extract magic byte detection into internal/magic package (#42)

## Summary

Extract magic byte detection and MIME type handling from `internal/imgcache/` into a new focused `internal/magic/` package.

Part of [issue #39](#39)

## Changes

### New package: `internal/magic/`

Moved the following from `internal/imgcache/magic.go`:
- `MIMEType` type and constants (`MIMETypeJPEG`, `MIMETypePNG`, etc.)
- `DetectFormat()` — detects image format from magic bytes
- `ValidateMagicBytes()` — validates content matches declared MIME type
- `PeekAndValidate()` — reads minimum bytes, validates, returns combined reader
- `IsSupportedMIMEType()` — checks if a MIME type is supported
- `MIMEToImageFormat()` — converts MIME type to ImageFormat
- `ImageFormatToMIME()` — converts ImageFormat to MIME string
- All error sentinels (`ErrUnknownFormat`, `ErrMagicByteMismatch`, `ErrNotEnoughData`)
- All helper functions (`detectSVG`, `skipBOM`, `normalizeMIMEType`)

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

### Updated imports
- `internal/imgcache/service.go`: uses `magic.ValidateMagicBytes()`
- `internal/imgcache/service_test.go`: uses `magic.DetectFormat()` and `magic.MIMEToImageFormat()`

### Naming
- Clean package-qualified names: `magic.DetectFormat()`, `magic.ValidateMagicBytes()`, etc.
- No stuttering names

### Tests
- Full test suite moved to `internal/magic/magic_test.go` (all 15 test functions preserved)
- All existing tests pass unchanged
- `docker build .` passes (includes `make check`: fmt, lint, tests)

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #42
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/imgcache/service.go

@@ -13,6 +13,7 @@ import (
 	"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.
@@ -277,7 +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)
 	}
 

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"