Ingest daily activity summaries

Up to 500 items per request. Each item carries a stable `client_id` partners own; replays return `duplicate` rather than creating a second sample. Partner-sourced samples are stamped server-side with `provider: partner:{tenant_slug}` and de-duplicated against OAuth/native data on read. Use `is_backfill=true` for historical batch loads to suppress real-time downstream side-effects.

curl -X POST "https://loading/v1/users/497f6eca-6276-4993-bfeb-53cbbbba6f08/metrics/activity/summary" \  -H "Content-Type: application/json" \  -d '{    "data": [      {}    ]  }'

{
  "data": [
    {
      "client_id": "string",
      "status": "accepted",
      "sample_id": "c94907f0-0a7b-412b-9563-59396ce4afee",
      "error": {}
    }
  ]
}

{
  "detail": [
    {
      "loc": [
        "string"
      ],
      "msg": "string",
      "type": "string"
    }
  ]
}