fix(config): require PLATFORM_URL explicitly, no hardcoded domain default

The previous fix defaulted PLATFORM_URL to this deployment's own domain
directly in docker-compose.prod.yml — any other deployment of this repo
would silently inherit it if they forgot to set their own. Now there is
no default at all: the compose file requires the env var to be set, and
the backend refuses to start in production if PLATFORM_URL still equals
the dev value, mirroring the existing SECRET_KEY enforcement.

docker-compose.prod.yml

@@ -91,8 +91,9 @@ services:
       REDIS_CACHE_DB: ${REDIS_CACHE_DB:-2}
       CORS_ORIGINS: ${CORS_ORIGINS:-}
       # Base URL used to build links in outbound emails (set-password, etc).
-      # Must be the real public frontend URL, not the dev default.
-      PLATFORM_URL: ${PLATFORM_URL:-http://aegis.undiamagico.es}
+      # No default on purpose — every deployment has a different domain;
+      # the backend refuses to start in production without this set.
+      PLATFORM_URL: ${PLATFORM_URL:?Set PLATFORM_URL in your .env file to this deployment's real public frontend URL}
       AEGIS_ENV: ${AEGIS_ENV:-production}
       SECURE_COOKIES: ${SECURE_COOKIES:-false}
       ADMIN_USERNAME: ${ADMIN_USERNAME:-admin}