Switches the GetChannelEvents query from a timestamp-ordered scan to an
id-keyset cursor (WHERE id > $cursor ORDER BY id ASC LIMIT $n). The
keyset cursor is stable under concurrent inserts and survives a future
retention job that prunes the oldest rows: a positional OFFSET would
silently skip events whenever rows below the cursor are deleted, while
"id > $cursor" keeps advancing past whatever the caller has already
seen. The id field is documented in the proto as a server-assigned
monotonic identity, so callers persist last_id as their sync watermark.
Adds a (channel_id, id) composite index to back the new query; the
existing (channel_id, timestamp) index does not cover it and would
force a per-channel filter after a global id scan.
We want to know if an update came from an initial sync. This also helps
us to identify data gaps and one can be sure it was not due to an actual
event. Modify the migration as it's unreleased.
