docs: IMAP deployment guides for all runtime targets

[ssilvius]

Goal

Documentation covering IMAP server deployment across all supported platforms.

Guides Needed

@rafters/mail-imap-cloudflare

• Durable Object setup, wrangler.toml config
• WebSocket transport, hibernation behavior
• D1 + R2 wiring for adapters
• Cost model (hibernated DOs are near-zero)

@rafters/mail-imap-server (covers all process-based runtimes)

Each guide uses the same Node TCP server package with platform-specific deployment:

• Fly.io: fly.toml, TCP passthrough, auto-scaling by connections, persistent volumes for state
• Railway: Dockerfile, TCP service, custom domain with port 993
• AWS Fargate: ECS task definition, ALB TCP listener on 993, ACM for TLS, CloudFormation/CDK
• Docker/VPS: docker-compose, direct TCP, Let's Encrypt for TLS
• Deno Deploy: NOT SUPPORTED (no persistent connections). Use Deno in Docker instead.
• Vercel: NOT SUPPORTED (no persistent connections). Use Fly/Railway alongside Vercel frontend.

Each guide should include:

• Prerequisites (database, blob storage, auth)
• Minimal working config
• TLS/certificate setup for port 993
• Testing with a real IMAP client (Thunderbird, Apple Mail)
• Monitoring/health checks
• Cost expectations

Context

• Protocol layer is vendor-free: @rafters/mail-imap
• Cloudflare gets its own runtime adapter: @rafters/mail-imap-cloudflare
• Everything else uses: @rafters/mail-imap-server (Node TCP/TLS)
• Related: feat: @rafters/mail-imap-cloudflare -- Durable Object runtime adapter #55 (Cloudflare adapter), feat: IMAP Spectrum TCP transport for native clients #57 (Spectrum transport)