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/tasks.md

@@ -43,7 +43,7 @@ Status legend: `DONE` `IN_PROGRESS` `TODO`
 | 17 | Create question-answer evaluation dataset from insurance regulation document | DONE | 12 |
 | 18 | Implement HTML benchmark report generation system | DONE | 14 |
 | 19 | Design HTML report structure for experiment comparison and visualization | DONE | 14 |
-| 20 | Create background processing jobs for document ingestion and benchmarking | TODO | 12 |
+| 20 | Create background processing jobs for document ingestion and benchmarking | DONE | 12 |
 
 ## Phase 5 — Wiring + Verification