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)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitattributes

@@ -0,0 +1,7 @@
+# Normalize line endings
+* text=auto eol=lf
+
+# The embedded admin SPA is first-party but ships as a UI asset; exclude it from
+# GitHub's language statistics so the repo is correctly identified as Rust.
+web/** linguist-vendored
+docs/screenshots/** linguist-vendored