fix: empty MCPD_BACKUP_REPO crashes mcpd on healthz with ERR_HTTP_HEADERS_SENT

Two bugs: (1) empty string env var treated as enabled (use || instead of ??),
(2) health routes missing return reply causing double-send with onSend hook.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/mcpd/src/routes/health.ts

@@ -12,7 +12,7 @@ export function registerHealthRoutes(app: FastifyInstance, deps: HealthDeps): vo
     const status = dbOk ? 'healthy' : 'degraded';
     const statusCode = dbOk ? 200 : 503;
 
-    reply.code(statusCode).send({
+    return reply.code(statusCode).send({
       status,
       version: APP_VERSION,
       uptime: process.uptime(),
@@ -25,6 +25,6 @@ export function registerHealthRoutes(app: FastifyInstance, deps: HealthDeps): vo
 
   // Simple liveness probe
   app.get('/healthz', async (_request, reply) => {
-    reply.code(200).send({ status: 'ok' });
+    return reply.code(200).send({ status: 'ok' });
   });
 }

src/mcpd/src/services/backup/git-backup.service.ts

@@ -75,7 +75,7 @@ export class GitBackupService {
     private readonly prisma: PrismaClient,
     repoUrl?: string,
   ) {
-    this.repoUrl = repoUrl ?? process.env.MCPD_BACKUP_REPO ?? null;
+    this.repoUrl = repoUrl || process.env.MCPD_BACKUP_REPO || null;
   }
 
   get enabled(): boolean {