Mount a persisted caption-local-keys volume at /data and load the optional
secrets/caption-local.env in both Compose files. The relay now reads a single
origins-only secrets/relay.json holding the shared master secret instead of
per-room tokens, so drop secrets/relay-tokens.env. Rewrite the README, AGENTS and
configuration guide for per-user keys, invites and derived relay rooms.
Bump caption-local to 116c3d0 and captionninja to 9105535.