feat: add ARQ background worker to container

- Create app/core/worker.py with WorkerSettings for ARQ
- Register all job functions: reindex, index_*, embedding_batch, deep_analysis
- Update prestart.sh to start ARQ worker in background before uvicorn
- Worker runs with max_jobs=10, job_timeout=300s

prestart.sh

@@ -4,15 +4,13 @@
 #
 # Responsibilities:
 #   1. Run Alembic DB migrations (alembic upgrade head).
-#   2. Start uvicorn as PID 1 (so signals like SIGTERM are forwarded correctly).
+#   2. Start ARQ background worker in background.
+#   3. Start uvicorn as PID 1 (so signals like SIGTERM are forwarded correctly).
 #
 # Notes:
-#   - `set -e` ensures the container crashes loudly if migrations fail,
-#     rather than starting a broken app on an inconsistent schema.
-#   - `exec uvicorn ...` replaces the shell process with uvicorn, so uvicorn
-#     becomes PID 1 and receives Docker's SIGTERM directly for graceful shutdown.
-#   - `--workers 1` is intentional for v1: SQLite (dev) is single-threaded,
-#     and asyncpg (prod) scales fine with one worker + an async connection pool.
+#   - `set -e` ensures the container crashes loudly if migrations fail.
+#   - ARQ worker runs in background, uvicorn becomes PID 1.
+#   - `--workers 1` is intentional for v1.
 # =============================================================================
 
 set -e
@@ -21,6 +19,9 @@ echo "[prestart] $(date -u +%Y-%m-%dT%H:%M:%SZ) - Running alembic upgrade head..
 alembic upgrade head
 echo "[prestart] DB migrations completed successfully."
 
+echo "[prestart] Starting ARQ background worker..."
+arq app.core.worker.WorkerSettings &
+
 echo "[prestart] Starting uvicorn on 0.0.0.0:8000 (workers=1)..."
 exec uvicorn app.main:app \
     --host 0.0.0.0 \