fix: resolve 10 code review issues

High priority:
- Fix concurrent race condition for view_count/like_count (atomic update)
- Add route request ID tracking to prevent race conditions
- Filter get_joke by status=approved (no pending content leak)
- Add error feedback for like button

Performance:
- Optimize random joke query (avoid full table sort)
- Limit page_size max to 100 (DoS prevention)

Medium:
- Add localStorage quota error handling
- Handle empty AI response gracefully
- Fix generate content title extraction

Low:
- Add rejected_jokes to stats API
- Update dashboard to show rejected count

api/app/routers/generate.py

@@ -76,8 +76,12 @@ def generate_joke(
         raise HTTPException(status_code=500, detail=f"生成失败: {str(e)}")
 
 
-def _parse_response(raw: str) -> GenerateResponse:
+def _parse_response(raw: str | None) -> GenerateResponse:
     """解析 AI 返回内容，提取标题和内容"""
+    # 防御：处理空或 None 输入
+    if not raw or not raw.strip():
+        raise ValueError("AI 返回内容为空")
+
     title = ""
     content = raw
 
@@ -86,16 +90,26 @@ def _parse_response(raw: str) -> GenerateResponse:
         line = line.strip()
         if line.startswith("标题：") or line.startswith("标题:"):
             title = line.split("：", 1)[-1].split(":", 1)[-1].strip()
-            content = content.replace(line, "").strip()
+            # 只替换这一行，不要 replace 全局
+            lines = content.split("\n")
+            for i, l in enumerate(lines):
+                if l.strip() == line:
+                    lines[i] = ""
+                    break
+            content = "\n".join(lines).strip()
             break
 
-    # 如果没有提取到标题，取第一行或前20字
+    # 如果没有提取到标题，取第一行
     if not title:
         first_line = raw.split("\n")[0].strip()
         if first_line.startswith("标题"):
             first_line = first_line.split("：", 1)[-1].split(":", 1)[-1].strip()
         title = first_line[:30] if len(first_line) > 30 else first_line
 
+    # 防御：content 不能为空
+    if not content.strip():
+        content = "（内容生成失败，请重新生成）"
+
     return GenerateResponse(
         title=title or "生成的笑话",
         content=content.strip(),