Stream Batch Predictions Events

Stream batch-job progress snapshots as SSE.

Each frame is a BatchPredictionsEvent-shaped JSON payload (with the per-prediction field stripped server-side; clients fetch the canonical list via GET /results). Frames carry both the SSE id: (Redis stream msg id) and event: (the snapshot's event field — start, prediction, score, finish).

Resumption: clients automatically send Last-Event-ID on reconnect; we use it as the XREAD start id so no events are missed (provided the stream hasn't been trimmed past it).

Heartbeats every ~15s keep proxies from idling the connection out. Synthetic terminal: if Celery reaches a terminal state without a finish snapshot reaching Redis, emit one so clients don't hang.