Replace the shared CAPTION_API_KEY with an optional per-user registry.
keys.py stores 64-character keys and single-use invites as SHA-256 digests
and derives each user's source/output rooms and capability tokens from a
shared master secret. /relay-config now returns those per-key values,
/claim.html redeems invites, and /admin.html (gated by CAPTION_ADMIN_KEY)
mints invites and revokes keys. /health and the static pages are public.
kawaiipunk
Steve Seguin