Add recipient email delivery service for sealed capsule share links

[nottherealalanturing]

Description

Manual copy-link works, but email delivery increases hackathon demo value and usability.

Scope

• Add pluggable email adapter with dev stub mode
• Trigger recipient email on seal
• Store delivery status and retry-safe metadata
• Review key files: recipient module, delivery service

Acceptance Criteria

• Sealing a capsule can trigger an email delivery record
• Dev/local mode works without a real provider
• Delivery failures are visible to sender/admin

Dependencies

Blocked by: Phase 1 share link generation issue

Stack

Express