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)

src/providers/mod.rs

@@ -0,0 +1,46 @@
+//! Outgoing provider adapters. The [`Provider`] trait is the single call site for LLMs;
+//! adapters translate the canonical [`ChatRequest`] to and from each provider's API format.
+//!
+//! Most local servers (vLLM, llama.cpp, LM Studio, Ollama `/v1`) are OpenAI-compatible
+//! → [`openai::OpenAiProvider`] is sufficient for all of them.
+
+pub mod anthropic;
+pub mod openai;
+
+use crate::model::{ChatRequest, ChatResponse};
+use async_trait::async_trait;
+
+/// Model capabilities — checked at selection time (whether tools/vision are needed).
+#[derive(Clone, Debug, Default)]
+pub struct Caps {
+    pub tools: bool,
+    pub vision: bool,
+    pub streaming: bool,
+}
+
+impl Caps {
+    pub fn from_list(list: &[String]) -> Self {
+        Self {
+            tools: list.iter().any(|c| c == "tools"),
+            vision: list.iter().any(|c| c == "vision"),
+            streaming: true,
+        }
+    }
+}
+
+#[async_trait]
+pub trait Provider: Send + Sync {
+    /// The model_id from config (not the provider's model name).
+    fn id(&self) -> &str;
+
+    fn caps(&self) -> &Caps;
+
+    /// Cost of a request in USD — computed from price_in/price_out × tokens (for billing).
+    fn price(&self, prompt_tokens: u32, completion_tokens: u32) -> f64;
+
+    /// Non-streaming chat call.
+    async fn chat(&self, req: ChatRequest) -> crate::error::Result<ChatResponse>;
+
+    // TODO(v0.1): async fn chat_stream(...) -> Result<ChatStream> (SSE).
+    // TODO(v0.1): async fn embed(...) -> Result<EmbedResponse>.
+}