feat: add 180 questions (90 sc + 90 fb) for networking subtopics

Cover 9 subtopics from the computer networking documentation:
- http-handshake: TCP/TLS/HTTP2/HTTP3 handshakes
- http-connection-cost: connection resource overhead & million concurrency
- connection-pooling: pool reuse, HTTP/1.1 vs HTTP/2
- http-keepalive: Keep-Alive principle, head-of-line blocking
- keepalive-scenarios: when to enable/disable Keep-Alive
- domain-sharding: HTTP/1.1 hack, HTTP/2 obsolescence
- cdn: edge nodes, caching, DDoS protection
- cors: same-origin policy, preflight requests
- go-build-strip: -s -w flags, binary size optimization

topics/networking/http-keepalive/meta.json

@@ -0,0 +1,50 @@
+{
+  "slug": "http-keepalive",
+  "name": "Http Keepalive",
+  "description": "",
+  "tags": [
+    "Connection头",
+    "HTTP/1.0",
+    "HTTP/1.1",
+    "HTTP/2",
+    "HTTP/3",
+    "HTTP头",
+    "HTTP演进",
+    "Keep-Alive",
+    "Keep-Alive参数",
+    "RST",
+    "TCP",
+    "max",
+    "timeout",
+    "串行",
+    "内存",
+    "利弊",
+    "并发连接",
+    "文件描述符",
+    "服务器",
+    "权衡",
+    "浏览器",
+    "空闲连接",
+    "连接关闭",
+    "连接模型",
+    "队头阻塞",
+    "默认行为"
+  ],
+  "difficulty_range": [
+    2,
+    4
+  ],
+  "schema_version": "1.0.0",
+  "updated": "2026-09-02",
+  "question_files": [
+    "single_choice",
+    "fill_blank"
+  ],
+  "stats": {
+    "total": 20,
+    "by_type": {
+      "single_choice": 10,
+      "fill_blank": 10
+    }
+  }
+}