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/models.py

@@ -361,6 +361,13 @@ class PlatformConfig(Base):
     compliance_rules = Column(JSON, default=dict)
     default_format = Column(Text, nullable=True)
     is_active = Column(Boolean, default=True)
+    requires_image = Column(Boolean, default=False)
+    image_count_min = Column(Integer, default=0)
+    image_count_max = Column(Integer, default=0)
+    image_width = Column(Integer, default=0)
+    image_height = Column(Integer, default=0)
+    min_words = Column(Integer, default=0)
+    max_words = Column(Integer, default=0)
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), onupdate=func.now())
 
@@ -375,6 +382,13 @@ class PlatformConfig(Base):
             "compliance_rules": self.compliance_rules or {},
             "default_format": self.default_format,
             "is_active": self.is_active,
+            "requires_image": self.requires_image,
+            "image_count_min": self.image_count_min,
+            "image_count_max": self.image_count_max,
+            "image_width": self.image_width,
+            "image_height": self.image_height,
+            "min_words": self.min_words,
+            "max_words": self.max_words,
             "created_at": self.created_at.isoformat() if self.created_at else None,
             "updated_at": self.updated_at.isoformat() if self.updated_at else None,
         }