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

internal/musicbrainz/model.go

@@ -0,0 +1,46 @@
+package musicbrainz
+
+import "time"
+
+// ReleaseGroup represents a MusicBrainz Release Group entity.
+// This is the primary data model for the provider - we work with
+// Release Groups to minimize duplicates from different releases.
+type ReleaseGroup struct {
+	ID          string
+	Title       string
+	Type        string
+	Status      string
+	ArtistID    string
+	ArtistName  string
+	ReleaseDate string
+}
+
+// Artist represents a MusicBrainz artist entity.
+type Artist struct {
+	ID   string
+	Name string
+}
+
+// ExternalRelease is the normalized form stored in the database,
+// matching the external_releases table schema.
+type ExternalRelease struct {
+	RGID        string
+	ArtistID    string
+	Title       string
+	Type        string
+	ReleaseDate string
+	CachedAt    time.Time
+}
+
+// ParsedReleaseGroups holds the result of parsing a MusicBrainz
+// release-group list XML response.
+type ParsedReleaseGroups struct {
+	ReleaseGroups []ReleaseGroup
+	Count         int
+}
+
+// ParsedArtist holds the result of parsing a MusicBrainz artist lookup.
+type ParsedArtist struct {
+	ID   string
+	Name string
+}