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/render/vendor/github.com/cloudwego/base64x/.golangci.yaml

@@ -0,0 +1,37 @@
+# Options for analysis running.
+run:
+  # include `vendor` `third_party` `testdata` `examples` `Godeps` `builtin`
+  skip-dirs-use-default: true
+  skip-dirs:
+    - kitex_gen
+  skip-files:
+    - ".*\\.mock\\.go$"
+# output configuration options
+output:
+  # Format: colored-line-number|line-number|json|tab|checkstyle|code-climate|junit-xml|github-actions
+  format: colored-line-number
+# All available settings of specific linters.
+# Refer to https://golangci-lint.run/usage/linters
+linters-settings:
+  gofumpt:
+    # Choose whether to use the extra rules.
+    # Default: false
+    extra-rules: true
+  govet:
+    # Disable analyzers by name.
+    # Run `go tool vet help` to see all analyzers.
+    disable:
+      - stdmethods
+linters:
+  enable:
+    - gofumpt
+    - goimports
+    - gofmt
+  disable:
+    - errcheck
+    - typecheck
+    - deadcode
+    - varcheck
+    - staticcheck
+issues:
+  exclude-use-default: true