feat: 平台配置表扩展配图/字数字段，微信公众号正文插入配图，合规检查从DB读规则

- PlatformConfig模型新增requires_image、image_count_min/max、image_width/height、min_words/max_words
- schemas.py同步PlatformConfigBase/Create/Update/Response新字段
- initial_data.py为三大平台填充初始值（微信需配图、字数800-1500等）
- database.py添加新列ALTER TABLE迁移
- platforms.html重写编辑弹窗（正确字段名+配图/字数设置）
- writer.py微信公众号文章正文h1后插入<img>占位
- compliance_checker.py接受platform_config参数，从DB读取规则替代硬编码
- compliance_optimizer.py启动时加载DB平台配置传入checker

scripts/writer.py

@@ -351,6 +351,17 @@ class Writer:
 
         html = template.replace("{{TITLE}}", title).replace("{{DATE}}", TODAY).replace("{{GEN_TIME}}", GEN_TIME)
         html_content = _md_parser(adapted)
+
+        # WeChat: insert image placeholder at start of body
+        if platform == "wechat":
+            img_tag = '<p><img src="placeholder.jpg" alt="配图" style="width:100%;max-width:1080px;border-radius:8px;"></p>\n'
+            # Insert after <h1> if present, else prepend
+            h1_end = html_content.find('</h1>')
+            if h1_end != -1:
+                html_content = html_content[:h1_end + 5] + '\n' + img_tag + html_content[h1_end + 5:]
+            else:
+                html_content = img_tag + html_content
+
         html = html.replace("<!-- CONTENT -->", html_content)
 
         tags_html = self._get_platform_tags(platform)