feat: auto-create Resend audiences per blueprint

Removes RESEND_AUDIENCE_WAITLIST env var. capture_waitlist_email() now
derives audience name from request.blueprints[0] (e.g. waitlist-auth,
waitlist-suppliers), lazily creates it via Resend API on first signup,
and caches the ID in a new resend_audiences table. Zero config beyond
RESEND_API_KEY — adding @waitlist_gate to any new blueprint auto-creates
its audience on first use.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -6,6 +6,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+### Changed
+- Auto-create Resend audiences per blueprint: `capture_waitlist_email()` now derives the audience name from `request.blueprints[0]` (e.g., `waitlist-auth`, `waitlist-suppliers`) and lazily creates audiences via the Resend API on first use, caching IDs in a new `resend_audiences` table; removes `RESEND_AUDIENCE_WAITLIST` env var — only `RESEND_API_KEY` needed
+
 ### Added
 - Simple A/B testing with `@ab_test` decorator and Umami `data-tag` integration
 - SEO defaults in `base.html`: canonical, og:url, og:type, og:image (logo fallback), og:title, og:description, twitter:card — every page gets these automatically, child templates override as needed