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

platform/backend/app/schemas.py

@@ -263,6 +263,13 @@ class PlatformConfigBase(BaseModel):
     compliance_rules: Dict[str, Any] = {}
     default_format: Optional[str] = None
     is_active: bool = True
+    requires_image: bool = False
+    image_count_min: int = 0
+    image_count_max: int = 0
+    image_width: int = 0
+    image_height: int = 0
+    min_words: int = 0
+    max_words: int = 0
 
 
 class PlatformConfigCreate(PlatformConfigBase):
@@ -278,6 +285,13 @@ class PlatformConfigUpdate(BaseModel):
     compliance_rules: Optional[Dict[str, Any]] = None
     default_format: Optional[str] = None
     is_active: Optional[bool] = None
+    requires_image: Optional[bool] = None
+    image_count_min: Optional[int] = None
+    image_count_max: Optional[int] = None
+    image_width: Optional[int] = None
+    image_height: Optional[int] = None
+    min_words: Optional[int] = None
+    max_words: Optional[int] = None
 
 
 class PlatformConfigResponse(PlatformConfigBase):