Add CI workflow, admin frontend, Prometheus/Grafana monitoring, Cloudflare cache rules and worker, Nginx config, CONTRIBUTING.md and update docker-compose.yml

CONTRIBUTING.md

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+# Contributing to Linear Void
+
+Thank you for considering contributing to Linear Void!
+
+## Development Process
+
+1. **Read the Architecture First**: All design decisions are documented in `docs/architecture/README.md` (arc42 template) and the ADRs in `docs/architecture/adr/` (ADR-001 to ADR-007).
+2. **Follow the Structure**: Use the folder layout defined in ADR-002 (`frontend/`, `backend/playout/`, `backend/chat/`, `infrastructure/`, etc.).
+3. **Update Documentation Before Code**: Create or update an ADR before implementing significant changes.
+4. **Update TODO.md**: Plan tasks in `.github/instructions/TODO.md`, mark as completed, and move finished sections to `DONE.md`.
+5. **Commit Messages**: Reference the relevant ADR (e.g., "feat(playout): add R2 sync per ADR-001").
+6. **Test with Docker**: Run `docker compose up --build` to test changes.
+
+## Code Style
+
+- Go: Use standard formatting (`go fmt`).
+- TypeScript: ESLint and Prettier.
+- Tailwind: Follow the utility-first approach.
+
+## Pull Requests
+
+- Reference the related ADR in the PR description.
+- Ensure all tests pass and documentation is updated.
+- The CI workflow will validate normalization, linting, and Docker builds.
+
+See the ADRs for architectural decisions and `docs/architecture/README.md` for the big picture. Thank you for contributing!