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/database.py

@@ -53,6 +53,13 @@ def init_db():
             for table, col, typ in [
                 ("users", "org_id", "VARCHAR DEFAULT 'default'"),
                 ("topics", "org_id", "VARCHAR DEFAULT 'default'"),
+                ("platform_configs", "requires_image", "BOOLEAN DEFAULT FALSE"),
+                ("platform_configs", "image_count_min", "INTEGER DEFAULT 0"),
+                ("platform_configs", "image_count_max", "INTEGER DEFAULT 0"),
+                ("platform_configs", "image_width", "INTEGER DEFAULT 0"),
+                ("platform_configs", "image_height", "INTEGER DEFAULT 0"),
+                ("platform_configs", "min_words", "INTEGER DEFAULT 0"),
+                ("platform_configs", "max_words", "INTEGER DEFAULT 0"),
             ]:
                 try:
                     conn.execute(text(f"ALTER TABLE {table} ADD COLUMN IF NOT EXISTS {col} {typ}"))