Add inbound email capture via Cloudflare Email Worker

- Cloudflare Email Worker forwards hello@ and nate@ to Gmail and POSTs
  parsed MIME email to /api/inbound-email as admin inbox entries
- New server route authenticates via shared secret and deduplicates by
  Message-ID before storing inbound emails as contact submissions
- Admin inbox shows "email" badge for inbound messages; reply composer
  opens blank with auto-subject "Re: [original]" and signature preview
- Documents setup steps in cloudflare/email-worker.js and .env.example

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

server.js

@@ -48,6 +48,7 @@ import { register as registerStudyComments } from './server/routes/study-comment
 import { register as registerStudyCertificate } from './server/routes/study-certificate.js'
 import { register as registerEpisodeScripts } from './server/routes/episode-scripts.js'
 import { register as registerContact } from './server/routes/contact.js'
+import { register as registerInboundEmail } from './server/routes/inbound-email.js'
 import { register as registerQuestions } from './server/routes/questions.js'
 import { register as registerAnalytics } from './server/routes/analytics.js'
 import { register as registerDownloads } from './server/routes/downloads.js'
@@ -91,6 +92,7 @@ registerStudyComments(app)
 registerStudyCertificate(app)
 registerEpisodeScripts(app)
 registerContact(app)
+registerInboundEmail(app)
 registerQuestions(app)
 registerAnalytics(app)
 registerDownloads(app)