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/model_test.go

@@ -168,34 +168,3 @@ func TestParseReleaseGroups_WithStatus(t *testing.T) {
 		t.Errorf("ReleaseGroups[0].Status = %q, want %q", result.ReleaseGroups[0].Status, "Bootleg")
 	}
 }
-
-func TestParseArtist_Success(t *testing.T) {
-	data := []byte(`<?xml version="1.0" encoding="UTF-8"?>
-<metadata xmlns="http://musicbrainz.org/ns/mmd-2.0#">
-  <artist id="artist-uuid-1" type="Group">
-    <name>Pink Floyd</name>
-  </artist>
-</metadata>`)
-
-	result, err := ParseArtist(data)
-	if err != nil {
-		t.Fatalf("ParseArtist() error = %v", err)
-	}
-
-	if result.ID != "artist-uuid-1" {
-		t.Errorf("ParseArtist().ID = %q, want %q", result.ID, "artist-uuid-1")
-	}
-
-	if result.Name != "Pink Floyd" {
-		t.Errorf("ParseArtist().Name = %q, want %q", result.Name, "Pink Floyd")
-	}
-}
-
-func TestParseArtist_MalformedXML(t *testing.T) {
-	data := []byte(`this is not xml`)
-
-	_, err := ParseArtist(data)
-	if err == nil {
-		t.Fatal("ParseArtist() expected error for malformed XML, got nil")
-	}
-}