feat: universal LLM gateway with embedded multilingual admin console

- OpenAI-compatible gateway with intelligent routing via cortiq-router (allaigate)
- Embedded build-free admin SPA at /admin: 7 languages (en/ru/de/fr/es/zh/tr),
  light/dark themes, hot config reload (no restart)
- Admin API, per-request statistics + Prometheus /metrics, secret store, playground
- English documentation (primary) with Russian translations (*.ru.md)

CONTRIBUTING.md

@@ -0,0 +1,46 @@
+*Read this in [Русский](CONTRIBUTING.ru.md).*
+
+# Contributing to Cortiq Gateway
+
+Thank you for your interest in the project! This is an open gateway and we welcome contributions.
+
+## Getting started
+
+1. Read [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) — design overview and roadmap.
+2. Review [docs/PROTOCOLS.md](docs/PROTOCOLS.md) and [docs/ROUTING.md](docs/ROUTING.md).
+3. The current phase is **Phase 0** (skeleton). Good entry-point tasks are labeled
+   `good first issue`.
+
+## How to add...
+
+- **A new inbound protocol** — implement the `InboundProtocol` trait in `src/protocols/`,
+  translate to/from the canonical model (`ChatRequest`/`ChatResponse`), and add the
+  corresponding flag in `[protocols]`. Do not touch the providers.
+- **A new provider** — implement the `Provider` trait in `src/providers/`. Most local
+  servers are OpenAI-compatible, so reusing `openai.rs` is often sufficient.
+- **A new routing policy** — extend `src/routing.rs` and the `[routing.policy]` section.
+
+Core principle: N protocols × M providers communicate **only** through the canonical model —
+no direct protocol↔provider translations.
+
+## Style and checks
+
+```bash
+cargo fmt --all
+cargo clippy --all-targets -- -D warnings
+cargo test
+```
+
+- Match the style of the surrounding code.
+- Secrets go only through `*_env` variables — never commit them to the repository.
+- New functionality must be accompanied by tests and updates to `docs/`.
+
+## Pull requests
+
+- Keep PRs small and focused. One PR = one logical unit of change.
+- In the description: what, why, and how it was tested.
+- Link the related issue if one exists.
+
+## License
+
+Contributions are accepted under [Apache-2.0](LICENSE).