feat(flags): the registry and a fail-closed Unleash client

Unleash holds flag state; it does not hold the list of flags. REGISTRY
is that list, because the SDK evaluates an unknown flag to False and
without a registry that is an undeclared False — indistinguishable from
a typo in a flag name.

Fail-closed throughout, and never raises: an unset UNLEASH_URL, an
unreachable server, a client that throws, an undeclared name — all
False. A flag layer that can 500 a request path or stop the API booting
is worse than one that is switched off.

Every flag defaults to False, with no per-flag override, because a flag
that defaults on is a kill switch and this is deliberately not one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

requirements.txt

@@ -12,4 +12,4 @@ slowapi==0.1.9
 secure==0.3.0
 typesense==0.21.0
 numpy==1.26.4
-
+UnleashClient==6.0.1