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>

scripts/init-db.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+# FlatRender V2 — run all schema migrations in order on first postgres init.
+# Mounted at: /docker-entrypoint-initdb.d/00-init.sh
+# Migrations dir mounted at: /migrations  (read-only)
+set -e
+
+MIGRATIONS_DIR="/migrations"
+
+echo "==> FlatRender: running schema migrations from $MIGRATIONS_DIR"
+
+for f in $(ls "$MIGRATIONS_DIR"/*.sql | sort); do
+    echo "  Applying: $(basename "$f")"
+    psql -v ON_ERROR_STOP=1 \
+         --username "$POSTGRES_USER" \
+         --dbname   "$POSTGRES_DB" \
+         --file     "$f"
+done
+
+echo "==> FlatRender: all migrations applied."