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/notification/vendor/github.com/gin-gonic/gin/CONTRIBUTING.md

@@ -0,0 +1,13 @@
+## Contributing
+
+- With issues:
+  - Use the search tool before opening a new issue.
+  - Please provide source code and commit sha if you found a bug.
+  - Review existing issues and provide feedback or react to them.
+
+- With pull requests:
+  - Open your pull request against `master`
+  - Your pull request should have no more than two commits, if not you should squash them.
+  - It should pass all tests in the available continuous integration systems such as GitHub Actions.
+  - You should add/modify tests to cover your proposed code changes.
+  - If your pull request contains a new feature, please document it on the README.