docs: improve email-worker specification and add VerificationFailed status

- Fix spelling and grammar errors throughout email-worker.md
- Add DB structure and connection sections
- Add error handling for email send failures
- Add email subjects, sender information, and delivery details
- Add logging requirements section
- Add race condition handling guidelines
- Add email provider specification (Mailgun)
- Add EmailStatus.VerificationFailed enum value for failed verification emails

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

web-app/app/lib/db-types.ts

@@ -41,6 +41,8 @@ export enum EmailStatus {
     Unverified = "unverified",
     /** Email is not yet verified - a verification request has been sent */
     VerificationPending = "verification-pending",
+    /** sending of verification email failed */
+    VerificationFailed = "verification-failed",
     /** Email is verified and is in good standing: emails are being successfully delivered */
     Verified = "verified",
     /** Recepient has unsubscribed from receiving emails via link - no further emails will be sent */