Enable contact delivery for InkAds marketing site (POC-259)

[patoperpetua]

Summary

Make PostKit /contact production-ready for the InkAds static marketing site (singleton-sd/poc-inkads-marketing) so browser forms can submit venue/advertiser enquiries without embedding API credentials.

ClickUp: POC-259 — lead capture acceptance criteria.

Wave

Wave 0 (prerequisite for marketing contact wiring). Can run in parallel with marketing legal-page routes.

Scope

• Confirm inkads.poc.singletonsd.com is in PostKit ORIGINS and host email profiles (contactInboxAddress, fromAddress) match Forward Email / tenant config for InkAds.
• Publish the marketing contact template for the InkAds tenant (e.g. marketing.contact-us or InkAds-specific key) so /contact returns success instead of TEMPLATE_NOT_FOUND.
• Document the public API base URL InkAds should set as PUBLIC_POSTKIT_API_BASE_URL (prod Function App).
• Define PR-preview behaviour: same-origin inkads.poc.singletonsd.com/pr-preview/pr-* should either use DevelopmentEmailProvider (default) or document when EMAIL_ALLOW_PREVIEW_SEND=true is required for end-to-end preview testing.
• Verify /health and a smoke POST /contact from an allowlisted Origin.

Out of scope

• InkAds Astro form UI (marketing repo)
• Privacy/Terms page copy (marketing repo + ClickUp editorial)

Acceptance

• POST /contact from https://inkads.poc.singletonsd.com with valid { name, email, subject, message } accepts and delivers (or dev-captures in non-prod per policy).
• Rate limiting and CORS behave as documented in docs/guides/public-forms.md.
• Marketing repo issue can reference a stable PUBLIC_POSTKIT_API_BASE_URL value.

[patoperpetua]

ClickUp parent: POC-259. Blocks: singleton-sd/poc-inkads-marketing#72