feat: add Web UI artist detail, archive, and ignore actions

Implements Task 8: artist detail page (local albums + found-missing
with ignore buttons), ignored-releases archive with restore, and POST
handlers toggling ignore flags and ignore_singles. Adds ErrArtistNotFound
sentinel so callers can distinguish missing artists, and wires routes via
the enhanced ServeMux path wildcard.

internal/database/artist_settings_test.go

@@ -44,7 +44,7 @@ func TestGetArtistSettings_Found(t *testing.T) {
 	}
 }
 
-// TestGetArtistSettings_NotFound verifies that a missing artist returns sql.ErrNoRows.
+// TestGetArtistSettings_NotFound verifies that a missing artist returns ErrArtistNotFound.
 func TestGetArtistSettings_NotFound(t *testing.T) {
 	db, err := New(":memory:")
 	if err != nil {
@@ -53,8 +53,8 @@ func TestGetArtistSettings_NotFound(t *testing.T) {
 	defer db.Close()
 
 	_, err = GetArtistSettings(db, "nonexistent")
-	if err != sql.ErrNoRows {
-		t.Errorf("expected sql.ErrNoRows, got %v", err)
+	if err != ErrArtistNotFound {
+		t.Errorf("expected ErrArtistNotFound, got %v", err)
 	}
 }