Accumulating milestone branch. One squashed commit per closed issue; `next` is kept green and mergeable to `main` at any time without notice.
Landed so far:
- `chore: update golangci-lint to v2.12.2 with canonical config` (#54) — canonical v2-schema `.golangci.yml`, pins bumped in `Dockerfile` and `script/bootstrap`, tree at `0 issues.`. Three behaviour deltas are recorded in that PR's body: `Cache.StoreVariant` takes a context, `MetadataStorage.Store` no longer leaks temp files on failure, and the `signing_key` too-short error text gained a `value too short:` prefix.
Sequencing for the milestone is tracked in #103.
Reviewed-on: #105
Co-authored-by: clawbot <clawbot@noreply.example.org>
## 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>
clawbot
