As a buyer, I want an API to upload my user IDs matched to TS synthetic IDs in bulk #266

Story

As a buyer,
I want an API to upload my user IDs matched to TS synthetic IDs in bulk,
so that TS can include my IDs in future bid requests for those users.

Parent Epic: #265

Acceptance Criteria

• Buyer can POST a batch of {synthetic_id, bidder, buyer_id} records
• API validates the payload (required fields, max batch size)
• Mappings are persisted in a KV store for fast lookup at auction time
• API is protected by authentication (e.g., Basic Auth)
• API returns a response indicating how many records were accepted and any errors

Example Request

POST /integrations/id_sync/upload
{
  "records": [
    { "synthetic_id": "abc123...", "bidder": "appnexus", "buyer_id": "buyer-uid-789" },
    { "synthetic_id": "def456...", "bidder": "rubicon", "buyer_id": "buyer-uid-012" }
  ]
}

Example Response

{
  "accepted": 2,
  "errors": []
}

Open Questions

• What is the maximum batch size per request?
• Should there be rate limiting per buyer?
• What authentication method? (Basic Auth, API key, mTLS?)