feat(jobs): add background job support for benchmarks and processing

Why:
- Long benchmark and process runs block HTTP clients; task #20 required async execution.

Changes:
- SQLite jobs table + GET /jobs and GET /jobs/{id}
- POST /benchmarks?background=true and POST /documents/{id}/process?background=true return 202 + job_id
- FastAPI BackgroundTasks execute work in-process; sync paths unchanged

Impact:
- Jobs are lost on server restart (Option A, no external queue)

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/api-reference.md

@@ -82,6 +82,9 @@ Delete a document and its vectors.
 
 Run chunking strategies on a document.
 
+**Query parameters:**
+- `background` (bool, default `false`) — when `true`, enqueue processing and return **202** with a `job_id`; poll `GET /jobs/{job_id}`.
+
 **Request:**
 ```json
 {
@@ -199,6 +202,9 @@ Retrieve a past query.
 
 Run a benchmark comparing multiple strategies.
 
+**Query parameters:**
+- `background` (bool, default `false`) — when `true`, enqueue the run and return **202** with a `job_id`; poll `GET /jobs/{job_id}` for status and result.
+
 **Request:**
 ```json
 {
@@ -278,6 +284,32 @@ List all experiments.
 
 ---
 
+### Jobs
+
+Background execution for long-running benchmarks and document processing (FastAPI `BackgroundTasks` + SQLite job records).
+
+#### `GET /jobs`
+
+List jobs (newest first).
+
+**Query parameters:** `job_type`, `status`, `offset`, `limit`
+
+#### `GET /jobs/{job_id}`
+
+Poll job status. When `status` is `completed`, `result` contains the same payload as the synchronous endpoint would return; when `failed`, `error` is set.
+
+**Response (202 enqueue body from POST with `background=true`):**
+```json
+{
+  "job_id": "abc123",
+  "job_type": "benchmark",
+  "status": "pending",
+  "poll_url": "/jobs/abc123"
+}
+```
+
+---
+
 ## Error Responses
 
 All errors return: