chore: JWT默认密钥改为35字节满足HMAC-SHA 256最低要求

cfc-backend/src/main/resources/application.yml

@@ -49,7 +49,7 @@ mybatis-plus:
       logic-not-delete-value: 0
 
 jwt:
-  secret: ${JWT_SECRET:cfc-dev-only-jwt-secret-2026}
+  secret: ${JWT_SECRET:cfc-prod-jwt-secret-2026-keep-it-safe!}
   expiration: 86400000
 
 zkxz: