fix(staging): export NITRO_* env-mapping for BREVO_API_KEY + hook secrets

start-staging.sh re-exports Infisical secrets als NITRO_*-prefixed envs
damit Nitro's runtimeConfig sie zur Laufzeit overrided (Default ist
Build-Time evaluated, also "" wenn nicht im Runner-Env).

Drei neue Mappings:
- BREVO_API_KEY → NITRO_BREVO_API_KEY
- HOOK_SEND_EMAIL_SECRETS → NITRO_HOOK_SEND_EMAIL_SECRETS
- MAIL_SENDER_EMAIL → NITRO_MAIL_SENDER_EMAIL

Ohne diese Mapping bleibt config.brevoApiKey leer im laufenden Backend →
send-email Hook fired 500 (siehe Smoke-Test).

backend/start-staging.sh

@@ -70,6 +70,9 @@ exec infisical run \
     [[ -n "${STRIPE_SECRET_KEY:-}" ]] && export NITRO_STRIPE_SECRET_KEY="$STRIPE_SECRET_KEY"
     [[ -n "${STRIPE_WEBHOOK_SECRET:-}" ]] && export NITRO_STRIPE_WEBHOOK_SECRET="$STRIPE_WEBHOOK_SECRET"
     [[ -n "${STRIPE_PUBLISHABLE_KEY:-}" ]] && export NITRO_PUBLIC_STRIPE_PUBLISHABLE_KEY="$STRIPE_PUBLISHABLE_KEY"
+    [[ -n "${BREVO_API_KEY:-}" ]] && export NITRO_BREVO_API_KEY="$BREVO_API_KEY"
+    [[ -n "${HOOK_SEND_EMAIL_SECRETS:-}" ]] && export NITRO_HOOK_SEND_EMAIL_SECRETS="$HOOK_SEND_EMAIL_SECRETS"
+    [[ -n "${MAIL_SENDER_EMAIL:-}" ]] && export NITRO_MAIL_SENDER_EMAIL="$MAIL_SENDER_EMAIL"
 
     exec '"$NODE_BIN"' '"$INDEX_MJS"'
   '