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/gateway/vendor/github.com/goccy/go-json/Makefile

@@ -0,0 +1,39 @@
+PKG := github.com/goccy/go-json
+
+BIN_DIR := $(CURDIR)/bin
+PKGS := $(shell go list ./... | grep -v internal/cmd|grep -v test)
+COVER_PKGS := $(foreach pkg,$(PKGS),$(subst $(PKG),.,$(pkg)))
+
+COMMA := ,
+EMPTY :=
+SPACE := $(EMPTY) $(EMPTY)
+COVERPKG_OPT := $(subst $(SPACE),$(COMMA),$(COVER_PKGS))
+
+$(BIN_DIR):
+	@mkdir -p $(BIN_DIR)
+
+.PHONY: cover
+cover:
+	go test -coverpkg=$(COVERPKG_OPT) -coverprofile=cover.out ./...
+
+.PHONY: cover-html
+cover-html: cover
+	go tool cover -html=cover.out
+
+.PHONY: lint
+lint: golangci-lint
+	$(BIN_DIR)/golangci-lint run
+
+golangci-lint: | $(BIN_DIR)
+	@{ \
+		set -e; \
+		GOLANGCI_LINT_TMP_DIR=$$(mktemp -d); \
+		cd $$GOLANGCI_LINT_TMP_DIR; \
+		go mod init tmp; \
+		GOBIN=$(BIN_DIR) go install github.com/golangci/golangci-lint/cmd/golangci-lint@v1.48.0; \
+		rm -rf $$GOLANGCI_LINT_TMP_DIR; \
+	}
+
+.PHONY: generate
+generate:
+	go generate ./internal/...