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.

.env.example

@@ -41,8 +41,9 @@ FRONTEND_PORT=80
 
 # ── Emails ────────────────────────────────────────────────────────────────────
 # Base URL used to build links in outbound emails (set-password, etc).
-# Must match your real public frontend URL — the app falls back to
-# http://localhost:5173 (dev default) if this is not set.
+# REQUIRED in production — must be THIS deployment's real public frontend
+# URL. There is no safe default (it's unique per deployment); the backend
+# refuses to start without it when AEGIS_ENV=production.
 PLATFORM_URL=https://your-domain.com
 
 # ── Environment flag ─────────────────────────────────────────────────────────