fix: Allow nullable ReplayGain and support 0.0 (#4239)

* fix(ui,scanner,subsonic): Allow nullable replaygain and support 0.0

Resolves #4236.

Makes the replaygain columns (track/album gain/peak) nullable.
Converts the type to a pointer, allowing for 0.0 (a valid value) to be returned from Subsonic.
Updates tests for this behavior.

* small refactor

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>
Co-authored-by: Deluan <deluan@navidrome.org>

db/migrations/20250701010104_make_replaygain_fields_nullable.go

@@ -0,0 +1,49 @@
+package migrations
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/pressly/goose/v3"
+)
+
+func init() {
+	goose.AddMigrationContext(upMakeReplaygainFieldsNullable, downMakeReplaygainFieldsNullable)
+}
+
+func upMakeReplaygainFieldsNullable(ctx context.Context, tx *sql.Tx) error {
+	_, err := tx.ExecContext(ctx, `
+ALTER TABLE media_file ADD COLUMN rg_album_gain_new real;
+ALTER TABLE media_file ADD COLUMN rg_album_peak_new real;
+ALTER TABLE media_file ADD COLUMN rg_track_gain_new real;
+ALTER TABLE media_file ADD COLUMN rg_track_peak_new real;
+
+UPDATE media_file SET
+	rg_album_gain_new = rg_album_gain,
+	rg_album_peak_new = rg_album_peak,
+	rg_track_gain_new = rg_track_gain,
+	rg_track_peak_new = rg_track_peak;
+
+ALTER TABLE media_file DROP COLUMN rg_album_gain;
+ALTER TABLE media_file DROP COLUMN rg_album_peak;
+ALTER TABLE media_file DROP COLUMN rg_track_gain;
+ALTER TABLE media_file DROP COLUMN rg_track_peak;
+
+ALTER TABLE media_file RENAME COLUMN rg_album_gain_new TO rg_album_gain;
+ALTER TABLE media_file RENAME COLUMN rg_album_peak_new TO rg_album_peak;
+ALTER TABLE media_file RENAME COLUMN rg_track_gain_new TO rg_track_gain;
+ALTER TABLE media_file RENAME COLUMN rg_track_peak_new TO rg_track_peak;
+	`)
+
+	if err != nil {
+		return err
+	}
+
+	notice(tx, "Fetching replaygain fields properly will require a full scan")
+	return nil
+}
+
+func downMakeReplaygainFieldsNullable(ctx context.Context, tx *sql.Tx) error {
+	// This code is executed when the migration is rolled back.
+	return nil
+}