Repeated Contacts syncs must never create duplicate birthdays. Enforce it on
the server, independent of the client's reconcile:
- POST /local/events is now idempotent on external_uid: if an event with the
same (calendar_id, external_uid) exists, update it in place instead of
inserting a new row
- startup cleanup removes existing duplicates sharing the same
(calendar_id, external_uid), keeping the earliest — auto-heals old data
- unique DB index on (calendar_id, external_uid) as a hard guarantee
external_uid is the stable "contact:<deviceId>:<contactId>", so the same contact
from the same device always maps to exactly one event.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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