refactor: update encryption key placeholder in environment sample

Author: opbot-xd

src/backend/.env.sample

@@ -8,7 +8,7 @@ SENTRY_DSN=...
 FRONTEND=...
 ADMIN_LIST=admin1|admin2
 MEMORY_LIMIT=500m
-ENCRYPTION_KEY=your_secure_random_base64_encryption_key_here
+ENCRYPTION_KEY=...
 
 # Health Monitor Configuration
 PROMETHEUS_URL=http://prometheus:9090