Phase 37: Hot-swap async + Phase 38: Universal API skeleton

- JobTracker extended with JobType::ProfileActivation + Embed
- activate_profile returns job_id immediately, work spawns in background
- /v1/chat, /v1/usage, /v1/sessions endpoints (OpenAI-compatible)
- Langfuse trace integration (Phase 40 early deliverable)
- 12 gateway unit tests green, curl gates pass

crates/vectord/src/supervisor.rs

@@ -170,7 +170,7 @@ pub async fn run_supervised(
                         // Update job tracker
                         let elapsed = start_time.elapsed().as_secs_f32();
                         let rate = if elapsed > 0.0 { done as f32 / elapsed } else { 0.0 };
-                        tracker.update_progress(&jid, done, rate).await;
+                        tracker.update_embed_progress(&jid, done, rate).await;
 
                         // Update checkpoint
                         let mut ckpt = ckpt.write().await;