Returns the project-wide referral bridge for ingestion pipelines.

Each user_referrers row (L1 edge) is returned as one result. Level 1 is referrer_identifier /
referrer_identifier_type. Levels 2–4 are nested under referral_chain (referrer_2–referrer_4), derived by
walking the current user_referrers chain within the project. Missing upstream referrers are null. Cycles
truncate the chain.

Authentication:

• Requires API key with service_role scope in the Authorization header: Authorization: Bearer <api_key>
• The project is automatically derived from the API key's associated project

Pagination:

• limit controls how many base user_referrers rows are returned per request (default 500, max 1000)
• after_id is a cursor on the base row UUID (user_referrer_id)
• count equals the number of base rows in results

Incremental loads:

• created_since filters base rows with created_at > value
• updated_since filters base rows with updated_at > value
• Ancestor user_referrers changes can alter derived L2–L4 values without updating descendant updated_at;
  periodic full backfills are recommended