feat: expose job start/stop times

[owbone]

Expose the start and stop times of jobs via the Job API and the lightning jobs list command. This will allow users to clean up old jobs.

⚡ lightning jobs list --teamspace general --sort-by stopped
┏━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━┳━━━━━━━━━━━━┓
┃ Name              ┃ Teamspace         ┃ Studio            ┃ Image             ┃ Status    ┃ Started          ┃ Stopped          ┃ Machine       ┃ Num Machines ┃ Total Cost ┃
┡━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━╇━━━━━━━━━━━━┩
│ nccl-sweep-0812   │ acme-ai/general   │                   │ nvcr.io/nvidia/p… │ Stopped   │ 2026-08-12 12:58 │                  │ H100_X_8      │ 4            │ 0.000      │
│ tokenizer-bench   │ acme-ai/general   │ misty-lake-42     │                   │ Completed │ 2026-08-24 19:53 │ 2026-08-24 19:55 │ CPU_SMALL     │ 1            │ 0.010      │
│ train-run-aug-25  │ acme-ai/general   │ dev-sandbox-2     │                   │ Failed    │ 2026-08-25 13:58 │ 2026-08-25 14:00 │ CPU_X_8       │ 1            │ 0.024      │
│ data-prep-retry   │ acme-ai/general   │ dev-sandbox-2     │                   │ Failed    │ 2026-08-25 14:23 │ 2026-08-25 14:25 │ CPU_X_8       │ 1            │ 0.030      │
└───────────────────┴───────────────────┴───────────────────┴───────────────────┴───────────┴──────────────────┴──────────────────┴───────────────┴──────────────┴────────────┘

⚡ lightning jobs list --teamspace general --json

[
  {
    "image": "python:3.11-slim",
    "machine": "CPU",
    "name": "quickstart-demo",
    "num_machines": 1,
    "started_at": "2026-07-22T20:24:28+00:00",
    "status": "Completed",
    "stopped_at": "2026-07-22T20:25:39+00:00",
    "studio": null,
    "teamspace": "acme-ai/general",
    "total_cost": 0.007
  },
  {
    "image": "nvcr.io/nvidia/pytorch:25.04-py3",
    "machine": "H100_X_8",
    "name": "nccl-check-2n",
    "num_machines": 2,
    "started_at": "2026-07-25T02:00:50.889629+00:00",
    "status": "Failed",
    "stopped_at": "2026-07-25T02:14:52.741917+00:00",
    "studio": null,
    "teamspace": "acme-ai/general",
    "total_cost": 0.0
  }
]