Rebrand: Co-op Check-in -> Co-op Survey in user-facing copy

Updates the tool product name across the app UI (header, page title,
section labels, form buttons, success/error states, report stat labels),
README, deployment docs, and the CiviCRM email template guidance.
Custom domain references move from check-in.fci.coop to survey.fci.coop
(DNS update still required).

The underlying CiviCRM "Check-in (organizing)" activity type, custom
group machine names (Check_in_data__organizing_), health-check ids,
and the internal org_engagement_check_in form id are unchanged --
those are CiviCRM contract surfaces, not product copy.

app/api/preview-link/route.ts

@@ -1,7 +1,7 @@
 /**
  * GET /api/preview-link?cid=<contactId>&token=<HEALTH_TOKEN>
  *
- * Returns a ready-to-send check-in URL for a given contact, plus enough
+ * Returns a ready-to-send survey URL for a given contact, plus enough
  * context for staff to verify it's the right org. Used for:
  *   - Generating links to paste into ad-hoc emails (or external tools)
  *   - Testing a contact's setup before sending the real CiviCRM email