Background job system for embedding — fixes 100K timeout

- JobTracker: create/update/complete/fail jobs with progress tracking
- POST /vectors/index now returns immediately with job_id (HTTP 202)
- Embedding runs in tokio::spawn background task
- GET /vectors/jobs/{id} returns live progress (chunks embedded, rate, ETA)
- GET /vectors/jobs lists all jobs
- Progress logged every 100 batches with chunks/sec and ETA
- 100K embedding job running successfully at 44 chunks/sec
- System stays responsive during embedding (queries in 23ms)

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

crates/gateway/src/main.rs

@@ -57,6 +57,7 @@ async fn main() {
         .nest("/vectors", vectord::service::router(vectord::service::VectorState {
             store: store.clone(),
             ai_client: ai_client.clone(),
+            job_tracker: vectord::jobs::JobTracker::new(),
         }))
         .nest("/workspaces", queryd::workspace_service::router(workspace_mgr));