UI: Live execution log viewer (SSE)

[AbirAbbas]

Ref #316, #319

The backend now exposes GET /api/ui/v1/executions/:id/logs/stream as an SSE endpoint that streams real-time execution lifecycle events. This data isn't surfaced in the UI yet.

What to add:

• Live log panel on the execution detail page
• Auto-scrolling event stream showing execution lifecycle (started, agent called, succeeded/failed)
• Error category and message displayed inline when execution fails
• Connection status indicator (connected/reconnecting)

SSE event shape:

{"type": "connected", "execution_id": "exec_123", "timestamp": "..."}
{"type": "execution_log", "level": "info", "message": "Agent called", ...}
{"type": "heartbeat", "timestamp": "..."}

In #316 the user noted there was no way to tell if an execution was stuck vs actively processing. This gives real-time visibility.