feat(web): auto-refresh projection when background job completes

[aaronsb]

Problem

When regenerating projections for large ontologies (≥500 concepts), the API queues a background job and returns immediately. The web client has no indication that it should poll for completion - users must manually refresh the page to see the new projection.

Current Behavior

1. User clicks regenerate on large ontology
2. API returns {"status": "queued", "job_id": "..."}
3. Job completes in 2-5 seconds
4. Web client shows stale/empty state
5. User must manually refresh to see results

Desired Behavior

1. User clicks regenerate
2. API returns job_id with queued status
3. Web client shows "Computing projection..." indicator
4. Client polls job status or projection endpoint
5. When complete, automatically loads new projection

Implementation Options

1. Polling: Client polls /projection/{ontology} every 2s until data arrives
2. Job status polling: Client polls /jobs/{job_id} until completed, then fetches projection
3. WebSocket/SSE: Push notification when job completes (more complex)

Option 1 (simple polling) is probably sufficient given projections complete in <10s.

Acceptance Criteria

• Web client detects when projection is queued vs computed
• Shows loading indicator during computation
• Automatically refreshes when projection becomes available
• Stops polling after timeout (30s) with error message