Sync generators blocked uvicorn's event loop preventing chunks from
being flushed. Converts to async generators with redis.asyncio so
streaming works correctly under ASGI. Adds /sse/ nginx location with
proxy_buffering off and 1h read timeout.
Without ?feed=, the endpoint subscribes to all notification channels via
Redis psubscribe("notifications:*") and adds target_feed to each event.
With ?feed=, behavior is unchanged. Updates README and root _links.
