feat: create MusicBrainz client and data models

Add internal/musicbrainz/ package with:
- client.go: MusicBrainzClient struct wrapping net/http.Client with
  channel-based rate limiter (1 req/sec), doGet method with proper
  User-Agent header, and Close for cleanup
- model.go: ReleaseGroup, Artist, ExternalRelease, and Parsed* structs
- XML parsing functions for release-group list and artist responses
- Comprehensive tests: XML parsing (success, empty, malformed),
  client constructor, doGet (success, non-200, unreachable server),
  rate limiter behavior

docs/plans/2026-05-21-musicbrainz-provider.md

@@ -47,13 +47,13 @@ Implement a MusicBrainz API provider with strict 1 request/second rate limiting,
 ## Implementation Steps
 
 ### Task 1: Create MusicBrainz client and data models
-- [ ] create `internal/musicbrainz/client.go` with MusicBrainzClient struct wrapping net/http.Client
-- [ ] implement constructor taking config and rate limiter
-- [ ] create `internal/musicbrainz/model.go` with structs for MusicBrainz API responses (ReleaseGroup, Artist, etc.)
-- [ ] implement XML parsing functions for MusicBrainz responses
-- [ ] write tests for XML parsing (success + error cases)
-- [ ] write tests for client constructor and basic API call structure
-- [ ] run tests - must pass before next task
+- [x] create `internal/musicbrainz/client.go` with MusicBrainzClient struct wrapping net/http.Client
+- [x] implement constructor taking config and rate limiter
+- [x] create `internal/musicbrainz/model.go` with structs for MusicBrainz API responses (ReleaseGroup, Artist, etc.)
+- [x] implement XML parsing functions for MusicBrainz responses
+- [x] write tests for XML parsing (success + error cases)
+- [x] write tests for client constructor and basic API call structure
+- [x] run tests - must pass before next task
 
 ### Task 2: Implement rate limiting and caching layer
 - [ ] add golang.org/x/time/rate dependency to go.mod