fix(email): wire PLATFORM_URL into prod compose, was always falling back to localhost

Set-password links in every password-setup/reset email pointed at
http://localhost:5173 in production because PLATFORM_URL was never mapped
in docker-compose.prod.yml's backend environment block — the app silently
used its dev default. Now defaults to the real domain from CORS_ORIGINS.

.env.example

@@ -39,6 +39,12 @@ CORS_ORIGINS=https://your-domain.com
 # ── Frontend ─────────────────────────────────────────────────────────────────
 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.
+PLATFORM_URL=https://your-domain.com
+
 # ── Environment flag ─────────────────────────────────────────────────────────
 # Set to "production" for production deployments (enforces SECRET_KEY, etc.)
 AEGIS_ENV=production