feat: V2 microservices stack — backend services, gateway, JWT auth

Add full V2 architecture: identity, content, studio (.NET 10) and file,
render, notification, gateway (Go) services with vendored deps, plus DB
migrations, event/API contracts, and an init-db script.

Wire the Next.js frontend to the gateway: server-side JWT auth routes
(login/register/refresh/logout/me), gateway fetch helper, and session/
cookie/jwt helpers under src/lib.

Containerize the stack via docker-compose.v2.yml and per-service
Dockerfiles. Base images resolve through a Nexus mirror (Docker Hub) and
MCR directly; npm/NuGet pull from Nexus groups. Self-host fonts via
next/font/local to avoid Google Fonts (geo-blocked).

Add CI workflow and ignore .env.v2, *.stackdump, and .NET bin/obj.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

services/identity/docker-compose.yml

@@ -0,0 +1,38 @@
+version: '3.9'
+
+services:
+  postgres:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_DB: flatrender
+      POSTGRES_USER: postgres
+      POSTGRES_PASSWORD: postgres
+    ports:
+      - "5432:5432"
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+      - ../../backend/db/migrations:/docker-entrypoint-initdb.d:ro
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U postgres"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+  identity:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    ports:
+      - "5010:8080"
+    environment:
+      ASPNETCORE_ENVIRONMENT: Development
+      ConnectionStrings__DefaultConnection: "Host=postgres;Port=5432;Database=flatrender;Username=postgres;Password=postgres;SearchPath=identity,public"
+      Jwt__Secret: "dev-jwt-secret-change-in-production-use-32-chars"
+      Jwt__Issuer: flatrender-identity
+      Jwt__Audience: flatrender
+    depends_on:
+      postgres:
+        condition: service_healthy
+
+volumes:
+  pgdata: