SSE connection limits — replace with polling or WebSocket

[howwohmm]

Problem

`main.py:214-247` — each enrollment opens an SSE stream that polls the DB every 5 seconds for up to 30 minutes. At 100 concurrent enrollments = 100 persistent connections, each hitting the DB every 5s.

Solution

• Replace SSE with client-side polling (exponential backoff: 2s → 5s → 10s → 30s)
• Or use WebSocket with server-push on actual state changes
• Add max concurrent SSE connections guard

Files

• `main.py` — SSE endpoint (lines 214-247)

Acceptance Criteria

• No long-lived connections per enrollment
• DB polling reduced by 90%+