POST /keys/query for a remote user who shares a room with a local user is
now answered from a cache, fetched whole with GET /user/devices/{userId}
on a miss. Incoming m.device_list_update EDUs are applied when their
prev_id matches the cached stream_id; anything else marks the copy stale
and it is fetched again in the background, and a stale copy is never
served. m.signing_key_update replaces the cached cross-signing keys. A
user's copy is dropped once no local user shares a room with them, by a
keyserver listener on the roomserver's emit stage.
Migration 0009 adds keyserver_remote_users, keyserver_remote_device_keys
and keyserver_remote_cross_signing_keys for sqlite and postgres.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
