Use connection pool for job enqueueing instead of per-request

- Added get_job_pool() for lazy pool initialization
- Added close_job_pool() for graceful shutdown
- Hooked pool shutdown into FastAPI lifespan
- Reuses connections instead of creating new ones per enqueue

backend/main.py

@@ -6,6 +6,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from database import Base, engine
+from jobs import close_job_pool
 from routes import audit as audit_routes
 from routes import auth as auth_routes
 from routes import availability as availability_routes
@@ -27,6 +28,8 @@ async def lifespan(app: FastAPI):
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
     yield
+    # Cleanup on shutdown
+    await close_job_pool()
 
 
 app = FastAPI(lifespan=lifespan)