As a TS operator, I want buyer ID mappings stored in a fast, edge-local KV store #270

Story

As a TS operator,
I want buyer ID mappings stored in a fast, edge-local KV store,
so that lookups during auction time add minimal latency.

Parent Epic: #265

Acceptance Criteria

• Mappings are stored in Fastly KV Store (sub-millisecond reads at the edge)
• Multiple buyer IDs can be stored per synthetic ID (one per bidder)
• Mappings can be overwritten (upsert behavior)
• The store is configurable (store name, enabled/disabled)

Open Questions

• Should mappings expire (TTL)? If so, what duration?
• What is the expected cardinality? (How many synthetic IDs x how many buyers?)
• Is there a need to delete/revoke individual mappings?