feat(center): enhance health check with DB readiness

Fabric.Backend.Center/src/common/health.controller.ts

@@ -1,9 +1,25 @@
-import { Controller, Get } from '@nestjs/common';
+import { Controller, Get, ServiceUnavailableException } from '@nestjs/common';
+import { DataSource } from 'typeorm';
 
 @Controller('healthz')
 export class HealthController {
+  constructor(private readonly dataSource: DataSource) {}
+
   @Get()
-  get() {
-    return { ok: true, service: 'center' };
+  async get() {
+    try {
+      await this.dataSource.query('SELECT 1');
+      return {
+        ok: true,
+        service: 'center',
+        database: 'ready',
+      };
+    } catch {
+      throw new ServiceUnavailableException({
+        ok: false,
+        service: 'center',
+        database: 'not_ready',
+      });
+    }
   }
 }