clawbot 6b4a1d7607 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>

2026-04-07 00:41:48 +02:00

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