fix: 重写routes_system.py修复API错误

完整重写系统监控API，确保：
- 正确的数据结构匹配JavaScript期待
- system.uptime/system.boot_time/system.process_count
- cpu.count/cpu.logical
- disk.free_gb
- network.sent_mb/recv_mb/sent_speed/recv_speed
- 所有异常处理和日志记录

修复500错误，现在系统监控应该正常显示

routes_system.py

@@ -7,6 +7,7 @@ import os
 import subprocess
 import time
 from datetime import datetime
+from flask import jsonify, request
 from utils import log_message, get_cpu_temperatures, load_alert_config
 
 try:
@@ -46,6 +47,7 @@ def register_system_routes(app):
             disk = psutil.disk_usage('/')
             disk_total_gb = round(disk.total / (1024**3), 2)
             disk_used_gb = round(disk.used / (1024**3), 2)
+            disk_free_gb = round(disk.free / (1024**3), 2)
             disk_percent = disk.percent
             
             # 网速（需要计算差值）
@@ -103,7 +105,7 @@ def register_system_routes(app):
                 'disk': {
                     'total_gb': disk_total_gb,
                     'used_gb': disk_used_gb,
-                    'free_gb': round(disk.free / (1024**3), 2),
+                    'free_gb': disk_free_gb,
                     'percent': disk_percent
                 },
                 'network': {
@@ -114,7 +116,8 @@ def register_system_routes(app):
                 }
             })
         except Exception as e:
-            return jsonify({'error': str(e)}), 500
+            log_message(f"系统监控API错误: {e}")
+            return jsonify({'error': str(e), 'available': False}), 500
     
     @app.route('/api/system/processes')
     def api_system_processes():
@@ -123,6 +126,9 @@ def register_system_routes(app):
             return jsonify({'error': 'psutil未安装'}), 500
         
         try:
+            limit = int(request.args.get('limit', 10))
+            limit = max(1, min(50, limit))
+            
             processes = []
             for proc in psutil.process_iter(['pid', 'name', 'cpu_percent', 'memory_percent', 'status']):
                 try:
@@ -138,7 +144,7 @@ def register_system_routes(app):
             
             # 按CPU排序
             processes.sort(key=lambda x: x['cpu'], reverse=True)
-            processes = processes[:50]  # 只返回前50个
+            processes = processes[:limit]
             
             return jsonify({'processes': processes})
         except Exception as e:
@@ -173,7 +179,6 @@ def register_system_routes(app):
     @app.route('/api/alerts', methods=['POST'])
     def api_save_alerts():
         """保存预警配置"""
-        from flask import request
         import json
         from config import ALERT_CONFIG_FILE