Investigate geographic placement: KR users served from LAX colo

[amondnet]

Sub-task of #3 (Fix D).

Workers Observability shows cf.country=KR / cf.colo=LAX for 100% of /api/* requests (~290ms RTT, paid ~once per page since calls are parallel).

Tasks:

• Determine why KR users hit LAX (likely *.workers.dev subdomain routing; test a zone custom domain).
• If moving the worker closer to users, add a nearby D1 read replica (D1 read replication / Sessions API) — otherwise multi-query endpoints get slower.
• Separate track from A/B/C.

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