feat: Anthropic provider/inbound + embeddings

- Anthropic outbound provider: /v1/messages, system extraction, x-api-key +
  anthropic-version; streaming translates Anthropic SSE -> unified OpenAI wire format
- Anthropic inbound adapter (POST /v1/messages): non-streaming JSON + streaming SSE
  (OpenAI -> Anthropic event translation)
- Embeddings: POST /v1/embeddings inbound + Provider::embed(); embedding models
  resolvable from the pool (registry no longer skips them)
- protocols mounted always, gated on live flags; admin meta marks them implemented

src/protocols/mod.rs

@@ -1,7 +1,9 @@
 //! Incoming protocol adapters. Each one builds its own axum routes and translates
 //! its protocol to/from the canonical model. Enabled via flags in `[protocols]`.
 
+pub mod anthropic_messages;
 pub mod openai_chat;
+pub mod openai_embeddings;
 
 use crate::state::SharedState;
 use axum::Router;
@@ -10,8 +12,9 @@ use axum::Router;
 /// each handler checks the live `protocols.*` flag from config itself, so
 /// toggles in the admin panel take effect without a restart (hot switching).
 pub fn build_router() -> Router<SharedState> {
-    Router::new().merge(openai_chat::routes())
-    // TODO: openai_completions, openai_embeddings, openai_models,
-    //       anthropic_messages, mcp, native_passthrough — add here as well,
-    //       with a check of the corresponding flag inside the handler.
+    Router::new()
+        .merge(openai_chat::routes())
+        .merge(openai_embeddings::routes())
+        .merge(anthropic_messages::routes())
+    // TODO: openai_completions, openai_models, mcp, native_passthrough.
 }