feat: add scoped per-mailbox agent access and remote MCP

[thbechtold]

Summary

Adds an opt-in, per-mailbox agent access layer for external automation clients. It provides scoped MCP and REST access without changing the existing dashboard or legacy MCP authorization model.

What changed

• Add named Agent Access keys with hashed-at-rest credentials and revocation.
• Scope each key to selected mailboxes and read, draft, and send permissions.
• Support draft-only and direct-send modes, test recipients, recipient allowlists, and daily limits.
• Add stateless Streamable HTTP MCP at /agent/mcp and JSON REST actions at /agent/api/<tool>.
• Add on-demand generate_reply_draft using the existing mailbox prompt and AI safety checks.
• Require an explicit footer choice for every content-writing operation and prevent duplicate footer insertion.
• Add idempotent write operations and explicit handling for uncertain provider outcomes.
• Add Settings UI, activity visibility, documentation, and focused tests.

Security and compatibility

• Existing dashboard, /api/*, /mcp, and mailbox data remain unchanged.
• No SQL migration, new bucket, or new Durable Object class is required.
• Agent keys cannot administer mailboxes or fall through to legacy routes.
• /agent/* is intended to be protected by a path-specific Cloudflare Access policy or Service Auth configuration.

Verification

• npm run test:agent-access (25 tests passing)
• npm run typecheck
• npm run build
• git diff --check

Existing email rows and storage are not rewritten or migrated.