Real-time change stream: subscribeChanges endpoint

[maxine-at-forecast]

Summary

Implement subscribeChanges — a WebSocket/SSE subscription endpoint that streams dataset/schema/lens/label change events, enabling downstream consumers to build real-time indexes.

Background

From the original XRPC design (forecast-bio/atdata#49):

• Output: Stream of change events for science.alt.dataset.* records
• Enables downstream consumers to build real-time indexes without polling

Open Questions

• Is this needed if consumers can use com.atproto.sync.subscribeRepos (the firehose) and filter for science.alt.dataset.* collections? Probably yes for convenience — the AppView can provide a filtered, enriched stream — but this is not a v1 blocker.
• WebSocket vs SSE transport
• Event format and replay/cursor semantics

Related

• Closed: Design XRPC query and procedure lexicons for ac.foundation.dataset AppView atdata#49 (XRPC lexicon design — listed as deferred)
• The AppView already consumes Jetstream internally; this would expose a similar stream to external clients