✨ feat(registration): add self-registration configuration option

- introduce ALLOW_SELF_REGISTRATION environment variable
- update .env-example and docker-compose.yml to include new setting
- modify registration logic to respect self-registration configuration

📝 docs(README): document self-registration configuration

- add description of ALLOW_SELF_REGISTRATION setting in README
- explain default value and its impact on user registration process

.env-example

@@ -5,3 +5,5 @@ SESSION_COOKIE_SECURE=1
 DATABASE_PATH=/app/data/fleetledger.db
 # Optional: Fernet key for encrypting management passwords (leave empty to disable)
 ENCRYPTION_KEY=
+# Allow self-registration after first admin (1 = allow, 0 = only admin-created)
+ALLOW_SELF_REGISTRATION=0