Poll one subscription's change feed

Limited, machine-only release. Returns the next ordered page of the durable civic change feed for one live feed. The resource filter is fixed by the subscription and cannot be overridden on the request: only cursor and limit are accepted. Without a cursor, polling starts at the start cursor captured when the feed was created; from then on the caller sends the cursor it stored after committing the previous page, exactly as on GET /api/v1/data/changes. The cursor stays an opaque position, not an authorization: it is bound to the subscription's resource filter, and it is derived from the account's feed position rather than from the key secret, so a new feed registered by a replacement key can resume from a retained position of the old feed while that position is still replayable. The stored start position carries no exemption from the same retention window. The server never advances the cursor and keeps no acknowledgement: a client advances only after its own work commits. The request is served only while the feed's owner, the presented key, and the account's paid eligibility are all current and the feed is unrevoked; otherwise the poll is refused before any read. Replay is retained for 30 days and an older cursor is refused rather than silently skipped.