Additive nullable column + unique index + createContactSubmissionIdempotent
(ON CONFLICT DO NOTHING, returns existing row, skips notify email on replay).
Route reads optional Idempotency-Key header. Lets the edge outbox replay a
contact submission without creating a duplicate. Backward-compatible: direct
submissions (no key) insert normally. touring/waitlist already natural-idempotent
(UNIQUE(email,provider_slug) upsert), so contact is the only table needing this.
NB: hCaptcha is effectively disabled (frontend sends no token), so stale-token
replays are not rejected; if hCaptcha is ever enabled, add a trusted outbox-token
bypass for replays.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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