Open sign-ups — waitlist gate removed, anyone can create an account

Features

• Open sign-ups — waitlist gate removed, anyone can create an account
• New agentic mailbox service with scoped authentication
• Mailbox registry for managing agent-connected mailboxes
• Inbound email events now tracked and threaded automatically
• Outbound email orchestration via the new request-send flow

Fixes

• Fixed cross-origin login failures by adding frontend to trusted origins
• Fixed route matching errors with Express 5 compatibility update