Segment sync freshness for a dashboard

Per-segment sync position for every segment a non-degraded panel pins. Calling it also FIRES the membership diff for those segments (rate limited to one per segment per 60s), so polling it while a dashboard is open is what self-drives change detection. The diff is fire and forget: the response reflects the PREVIOUS diff, so at the console's ~30s cadence a one-poll lag is deliberate. This endpoint must stay fast, and freshness is an accelerator - live queries are the correctness mechanism. seq is GREATEST(max(seq), prunedThroughSeq), so retention pruning never looks like a rewind. A segment never diffed (or a deployment without the sync tables) reads { seq: 0, lastChangeAt: null }.