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/prisma/schema.prisma

@@ -0,0 +1,16 @@
+datasource db {
+  provider = "postgresql"
+  url      = env("DATABASE_URL")
+}
+
+generator client {
+  provider = "prisma-client-js"
+}
+
+model Tenant {
+  id        String   @id @default(uuid())
+  name      String
+  slug      String   @unique
+  createdAt DateTime @default(now())
+  updatedAt DateTime @updatedAt
+}