fix: address code-review findings (startup validation, shutdown hooks, health check, deploy config)

Verified each finding against the real codebase first, since the review's
diff was polluted by a stale leftover agent-worktree copy under
.claude/worktrees/ (now removed) that made several already-fixed or
never-broken issues look current (CORS, missing typecheck script, PWA
icons, font CSS, Helmet/Swagger CSP - all confirmed fine live, not
touched).

Real fixes:
- main.ts: fail-fast on missing DATABASE_URL/JWT_ACCESS_SECRET at boot,
  call enableShutdownHooks() so PrismaService.onModuleDestroy() actually
  runs on SIGTERM during k8s rolling deploys, PORT fallback 3000 -> 3001
  to match every other place that documents the port.
- health.controller.ts: log the real underlying DB error instead of
  discarding it, add a 5s timeout to the readiness query so a hung DB
  fails the probe fast instead of blocking indefinitely.
- Frontend Dockerfile: declare ARG/ENV NEXT_PUBLIC_API_URL before the
  build step, since Next.js inlines NEXT_PUBLIC_* vars at build time and
  a runtime-only ConfigMap value never reaches the compiled bundle.
- Backend ingress.yaml: drop the no-op rewrite-target regex for a plain
  prefix path (identical behavior, less confusing).

ApiCore-Backend/src/main.ts

@@ -4,8 +4,22 @@ import { DocumentBuilder, SwaggerModule } from '@nestjs/swagger';
 import { AppModule } from './app.module';
 import { configureApp } from './app.config';
 
+const REQUIRED_ENV_VARS = ['DATABASE_URL', 'JWT_ACCESS_SECRET'] as const;
+
+function assertRequiredEnvVars(): void {
+  const missing = REQUIRED_ENV_VARS.filter((name) => !process.env[name]);
+  if (missing.length > 0) {
+    throw new Error(
+      `Missing required environment variable(s): ${missing.join(', ')}`,
+    );
+  }
+}
+
 async function bootstrap() {
+  assertRequiredEnvVars();
+
   const app = await NestFactory.create<NestExpressApplication>(AppModule);
+  app.enableShutdownHooks();
 
   configureApp(app);
 
@@ -18,6 +32,6 @@ async function bootstrap() {
   const swaggerDocument = SwaggerModule.createDocument(app, swaggerConfig);
   SwaggerModule.setup('api/v1/docs', app, swaggerDocument);
 
-  await app.listen(process.env.PORT ?? 3000);
+  await app.listen(process.env.PORT ?? 3001);
 }
 void bootstrap();