fix: address code review findings

Remove dead code: duplicate ExternalRelease/Artist/ParsedArtist structs in
model.go, ParseArtist/mbArtist/mbArtistData in client.go, ArtistTypeFilter
and related filtering functions in api.go, SyncArtistDiscographyWithFilter
in sync.go, and CacheStats/IsArtistCacheValid in cache.go.

Fix bugs: SaveExternalRelease now stores NULL instead of empty string for
zero CachedAt; sync upserts are now transactional with stale release cleanup;
getCachedReleases returns int instead of *CacheStats; doGet uses url.Values
for proper query encoding of MBID.

Fix tests: removed duplicate TestRun_GracefulShutdown, removed dead code
(_ = dbPath) from TestNewApp, fixed assertions in httptest handler goroutine
to avoid data race, increased rate limiter timing tolerance, removed
Client.Close() calls (no-op removed), fixed sync test cache expiry to use
UPDATE instead of 0 TTL races.

Fix formatting: cancel()}() formatting in main.go, error format string in sync.go.

internal/musicbrainz/client.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"net/url"
 	"time"
 
 	"golang.org/x/time/rate"
@@ -35,23 +36,6 @@ func NewClient(cfg config.MusicBrainzConfig) *MusicBrainzClient {
 	}
 }
 
-// NewClientWithLimiter creates a MusicBrainzClient with a custom rate limiter.
-// This is primarily used for testing to inject a mock rate limiter.
-func NewClientWithLimiter(cfg config.MusicBrainzConfig, rl *rate.Limiter) *MusicBrainzClient {
-	return &MusicBrainzClient{
-		httpClient: &http.Client{
-			Timeout: 30 * time.Second,
-		},
-		userAgent:   cfg.UserAgent,
-		baseURL:     "https://musicbrainz.org/ws/2",
-		rateLimiter: rl,
-	}
-}
-
-// Close is a no-op for the x/time/rate-based client (the limiter does not
-// spawn goroutines), but retained for API compatibility.
-func (c *MusicBrainzClient) Close() {}
-
 // doGet performs a rate-limited HTTP GET request to the MusicBrainz API.
 // It blocks until the rate limiter allows the request, then sets the proper
 // User-Agent header and returns the response body.
@@ -126,18 +110,6 @@ type mbReleaseGroupList struct {
 	ReleaseGroupList mbReleaseGroupListXML `xml:"release-group-list"`
 }
 
-// mbArtistData represents the artist element inside metadata.
-type mbArtistData struct {
-	ID   string `xml:"id,attr"`
-	Name string `xml:"name"`
-}
-
-// mbArtist represents the XML structure of a MusicBrainz artist response.
-type mbArtist struct {
-	XMLName xml.Name     `xml:"metadata"`
-	Artist  mbArtistData `xml:"artist"`
-}
-
 // ParseReleaseGroups parses a MusicBrainz release-group list XML response
 // into a ParsedReleaseGroups struct.
 func ParseReleaseGroups(data []byte) (*ParsedReleaseGroups, error) {
@@ -163,14 +135,7 @@ func ParseReleaseGroups(data []byte) (*ParsedReleaseGroups, error) {
 	return result, nil
 }
 
-// ParseArtist parses a MusicBrainz artist XML response into a ParsedArtist struct.
-func ParseArtist(data []byte) (*ParsedArtist, error) {
-	var artist mbArtist
-	if err := xml.Unmarshal(data, &artist); err != nil {
-		return nil, fmt.Errorf("parse artist XML: %w", err)
-	}
-	return &ParsedArtist{
-		ID:   artist.Artist.ID,
-		Name: artist.Artist.Name,
-	}, nil
+// buildPath constructs a properly URL-encoded query path for the MusicBrainz API.
+func buildPath(endpoint string, params url.Values) string {
+	return endpoint + "?" + params.Encode()
 }