phase 12+13: HTTP API server with OpenAI-compatible endpoint (Milestone ③)

New crate: xserv-server
- Engine thread: loads Qwen3-8B, processes requests sequentially
- axum HTTP server: /health, /v1/models, /v1/chat/completions
- tokio::sync::mpsc channel between API and engine threads
- Non-streaming JSON response (streaming SSE to be added later)

API is OpenAI-compatible:
  POST /v1/chat/completions {"messages": [...], "max_tokens": N}
  → {"choices": [{"message": {"content": "..."}}]}

Verified: "Hi" → ", I'm" (3 tokens), model runs correctly via HTTP.

Key learnings:
- std::sync::mpsc::SyncSender is Send but NOT Sync → wrap in Mutex for Arc<AppState>
- MutexGuard must not live across await points (scope carefully)
- axum 0.8 Extension<Arc<T>> requires T: Send + Sync

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/xserv-server/src/api.rs

@@ -0,0 +1,115 @@
+use axum::Extension;
+use axum::Json;
+use serde::{Deserialize, Serialize};
+use std::sync::Arc;
+use uuid::Uuid;
+
+use crate::engine::{GenerateEvent, GenerateRequest};
+use crate::AppState;
+
+#[derive(Deserialize)]
+pub struct ChatRequest {
+    #[serde(default)]
+    pub model: Option<String>,
+    pub messages: Vec<Message>,
+    #[serde(default = "default_max_tokens")]
+    pub max_tokens: usize,
+}
+
+#[derive(Deserialize)]
+pub struct Message {
+    pub role: String,
+    pub content: String,
+}
+
+fn default_max_tokens() -> usize { 256 }
+
+#[derive(Serialize)]
+pub struct ModelsResponse {
+    object: &'static str,
+    data: Vec<ModelInfo>,
+}
+
+#[derive(Serialize)]
+pub struct ModelInfo {
+    id: String,
+    object: &'static str,
+    owned_by: &'static str,
+}
+
+pub async fn health() -> &'static str { "ok" }
+
+pub async fn list_models(Extension(state): Extension<Arc<AppState>>) -> Json<ModelsResponse> {
+    Json(ModelsResponse {
+        object: "list",
+        data: vec![ModelInfo {
+            id: state.model_name.clone(),
+            object: "model",
+            owned_by: "xserv",
+        }],
+    })
+}
+
+pub async fn chat_completions(
+    Extension(state): Extension<Arc<AppState>>,
+    Json(req): Json<ChatRequest>,
+) -> Json<serde_json::Value> {
+    let id = format!("chatcmpl-{}", Uuid::new_v4());
+    let model_name = state.model_name.clone();
+    let created = std::time::SystemTime::now()
+        .duration_since(std::time::UNIX_EPOCH)
+        .unwrap()
+        .as_secs();
+
+    // Prepare prompt tokens (MutexGuard scoped)
+    let prompt = build_prompt(&req.messages);
+    let prompt_tokens = state.engine_tokenizer.lock().unwrap().encode(&prompt);
+
+    // Create channel and submit request (MutexGuard scoped)
+    let (tx, mut rx) = tokio::sync::mpsc::channel::<GenerateEvent>(64);
+    let gen_req = GenerateRequest {
+        prompt_tokens,
+        max_tokens: req.max_tokens,
+        sender: tx,
+    };
+    state.engine_sender.lock().unwrap().send(gen_req).unwrap();
+
+    // Now await — no MutexGuards held here
+    let mut content = String::new();
+    let mut finish_reason = "length".to_string();
+    while let Some(event) = rx.recv().await {
+        match event {
+            GenerateEvent::Token { text, .. } => content.push_str(&text),
+            GenerateEvent::Done { finish_reason: fr } => { finish_reason = fr; break; }
+        }
+    }
+
+    Json(serde_json::json!({
+        "id": id,
+        "object": "chat.completion",
+        "created": created,
+        "model": model_name,
+        "choices": [{
+            "index": 0,
+            "message": { "role": "assistant", "content": content },
+            "finish_reason": finish_reason,
+        }],
+        "usage": {
+            "prompt_tokens": 0,
+            "completion_tokens": 0,
+            "total_tokens": 0
+        }
+    }))
+}
+
+fn build_prompt(messages: &[Message]) -> String {
+    let mut prompt = String::new();
+    for msg in messages {
+        match msg.role.as_str() {
+            "system" => { prompt.push_str(&msg.content); prompt.push('\n'); }
+            "user" | "assistant" => { prompt.push_str(&msg.content); }
+            _ => {}
+        }
+    }
+    prompt
+}