As a publisher, I want TS to automatically look up and attach buyer IDs to bid requests #267

Story

As a publisher,
I want TS to automatically look up and attach buyer IDs to bid requests,
so that buyers can recognize users and submit higher-quality bids.

Parent Epic: #265

Acceptance Criteria

• Before sending an OpenRTB bid request to Prebid Server, TS looks up buyer IDs for the current user's synthetic ID
• If buyer IDs are found, they are included in the request using the IAB standard user.eids field (OpenRTB 2.6)
• Each bidder receives only their own buyer ID (via Prebid Server's EID routing)
• If no buyer IDs are found, the request proceeds normally (no degradation)
• KV lookup failures are logged but do not block or delay the auction

Expected Wire Format

{
  "user": {
    "id": "<synthetic_id>",
    "eids": [
      { "source": "appnexus", "uids": [{ "id": "buyer-uid-789", "atype": 3 }] }
    ]
  }
}