Add high-watermark support

This page is a capture in the next bucket of the product backlog — a pre-sprint idea, not yet pulled into a sprint as a story.

Whenever we request entities, we should also return the watermark for that entity. This is the highest timestamp of all entities within a set (across all pages). This is so we can redo the exact same query with an "as of" and get exactly the same result set. Do some analysis with Claude on how best to do this. Maybe we should always query as of so that we know what the watermark is up front.

Notes:

• once we have watermark, we need to add a staleness indicator to all dialogs. This will require thresholds. If the data is older than X the staleness indicator goes yellow, older than Y it goes red.