Stream emails via SSE

Opens a Server-Sent Events stream for the given inbox. The server polls Redis with an exponential backoff strategy and pushes an event the moment an email arrives. The connection closes automatically after the first email or after 30 seconds (timeout).

Backoff strategy:

• 0–2s: polls every 500ms
• 2–10s: polls every 1000ms
• 10s+: polls every 2000ms

Events:

• : connected — connection established
• : keepalive — heartbeat every 10s to prevent proxy timeout
• data: {email JSON} — email arrived
• data: __timeout__ — no email within 30s, connection closing

For Node.js environments, use fetch() with ReadableStream rather than EventSource (which is browser-only).