feat(01-01): NestJS API with health endpoint and Prisma schema

- NestJS app with ConfigModule and HealthModule
- GET /health endpoint returning {status, timestamp} using HealthResponse type
- Prisma schema with PostgreSQL datasource and Tenant model (multi-tenancy foundation)
- Multi-stage Dockerfile with non-root nestjs user, monorepo root as build context
- Workspace dependency on @tessera/shared for shared types
- Type-check passes successfully

apps/api/src/app.module.ts

@@ -0,0 +1,11 @@
+import { Module } from '@nestjs/common';
+import { ConfigModule } from '@nestjs/config';
+import { HealthModule } from './health/health.module';
+
+@Module({
+  imports: [
+    ConfigModule.forRoot({ isGlobal: true }),
+    HealthModule,
+  ],
+})
+export class AppModule {}