chore: enable registration env + bump backend/webui submodules

Add ALLOW_REGISTRATION + PUBLIC_ENABLE_REGISTRATION (default true) to
.env.example and bump both submodules to the registration-flow commits.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -32,6 +32,9 @@ REDIS_URL=redis://localhost:6379/0
 JWT_SECRET=change-me-in-prod
 ACCESS_TOKEN_TTL_SECONDS=900
 REFRESH_TOKEN_TTL_SECONDS=2592000
+# Public self-service sign-up (POST /auth/register). Set to false to make
+# accounts admin-only. Backend authority; pair with PUBLIC_ENABLE_REGISTRATION.
+ALLOW_REGISTRATION=true
 
 # media / storage (paths inside the container)
 MEDIA_PATH=/data/media
@@ -51,3 +54,5 @@ MUSICBRAINZ_USER_AGENT=mcma-backend/0.1.0 ( https://github.com/your/repo )
 # ---- Frontend (webui) ------------------------------------------------
 # Served same-origin behind nginx, so the default '/api/v1' just works.
 PUBLIC_API_BASE_URL=/api/v1
+# Show the public sign-up UI. Keep in sync with ALLOW_REGISTRATION above.
+PUBLIC_ENABLE_REGISTRATION=true