Add passkey configuration and availability policy (inert)

Groundwork for passkey authentication, with the feature switched off by
default and no behaviour change when it is off.

Decision D1: passkeys require central authentication. A passkey is scoped to
a relying party spanning the base domain, which only exists when
SUBDOMAIN_REDIRECT is on and AUTH_SUBDOMAIN resolves to a base domain. The
RP ID is therefore always that base domain, never the request host.

Decision D4: HTTPS is required and is not exemptible. The allowed origin is
built as https://{authSubdomain} from configuration and never from the
request, so an http:// origin cannot be accepted, and isAvailableFor()
additionally refuses to offer the UI on a non-secure connection. The
deprecated setSecuredRelyingPartyId() escape hatch is not used and there is
deliberately no override that could reintroduce one.

Enabling PASSKEY_ENABLED without a usable configuration is a hard error via
a non-optional cache warmer, because entrypoint.sh runs cache:warmup on every
production boot: a misconfigured deployment fails to start instead of
offering a button that cannot work.

Also drops 12 obsolete phpstan-baseline entries for TotpTestHelper: adding
#[\Override] to its anonymous clock removed the rule violation at its source
rather than suppressing it.

Suite: 333 tests / 770 assertions (was 313 / 738), 100% coverage on new
files. phpstan level 6 clean, php-cs-fixer clean, conformance 35/35.

phpunit.dist.xml

@@ -46,6 +46,8 @@
         </include>
 
         <deprecationTrigger>
+            <method>Doctrine\Deprecations\Deprecation::trigger</method>
+            <method>Doctrine\Deprecations\Deprecation::delegateTriggerToBackend</method>
             <function>trigger_deprecation</function>
         </deprecationTrigger>
     </source>