feat: Proxy extensions — /v1/completions + /v1/embeddings

- Added generic execute_proxy_request executor for any path suffix
- POST /v1/completions — forwards to upstream /completions
- POST /v1/embeddings — forwards to upstream /embeddings
- Both use existing auth + route resolution infra
- 18/18 tests pass

boosapi-rs/src/routes/proxy/mod.rs

@@ -45,6 +45,8 @@ pub enum DownstreamSource {
 pub fn routes() -> Router<AppState> {
     Router::new()
         .route("/v1/chat/completions", post(chat_completions))
+        .route("/v1/completions", post(completions_handler))
+        .route("/v1/embeddings", post(embeddings_handler))
         .route("/v1/images/generations", post(images_generations))
         .route("/v1/models", get(list_models))
 }
@@ -82,6 +84,48 @@ async fn chat_completions(
     }
 }
 
+/// POST /v1/completions
+async fn completions_handler(
+    State(state): State<AppState>,
+    headers: HeaderMap,
+    Json(body): Json<Value>,
+) -> AppResult<Json<Value>> {
+    info!("POST /v1/completions");
+
+    check_auth(&state, &headers).await?;
+
+    let model = body
+        .get("model")
+        .and_then(|m| m.as_str())
+        .ok_or_else(|| AppError::BadRequest("model is required".into()))?
+        .to_string();
+
+    let route = resolve_route(&state, &model).await?;
+    let result = executors::execute_proxy_request(state, route, "/completions", body).await?;
+    Ok(Json(result))
+}
+
+/// POST /v1/embeddings
+async fn embeddings_handler(
+    State(state): State<AppState>,
+    headers: HeaderMap,
+    Json(body): Json<Value>,
+) -> AppResult<Json<Value>> {
+    info!("POST /v1/embeddings");
+
+    check_auth(&state, &headers).await?;
+
+    let model = body
+        .get("model")
+        .and_then(|m| m.as_str())
+        .ok_or_else(|| AppError::BadRequest("model is required".into()))?
+        .to_string();
+
+    let route = resolve_route(&state, &model).await?;
+    let result = executors::execute_proxy_request(state, route, "/embeddings", body).await?;
+    Ok(Json(result))
+}
+
 /// POST /v1/images/generations
 async fn images_generations(
     State(state): State<AppState>,