feat: 平台规则从硬编码改为品牌方上传文档 + AI 解析

- 新增 PlatformRule 模型 (draft/active/inactive 状态流转)
- 新增文档解析服务 (PDF/Word/Excel → 纯文本)
- 新增 4 个 API: 解析/确认/查询/删除平台规则
- 脚本审核优先从 DB 读取 active 规则，硬编码兜底
- 视频审核合并平台规则违禁词到检测列表
- Alembic 迁移 006: platform_rules 表

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/alembic/versions/006_add_platform_rules.py

@@ -0,0 +1,38 @@
+"""添加平台规则表
+
+Revision ID: 006
+Revises: 005
+Create Date: 2026-02-10
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers, used by Alembic.
+revision: str = '006'
+down_revision: Union[str, None] = '005'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        'platform_rules',
+        sa.Column('id', sa.String(64), primary_key=True),
+        sa.Column('tenant_id', sa.String(64), sa.ForeignKey('tenants.id', ondelete='CASCADE'), nullable=False, index=True),
+        sa.Column('brand_id', sa.String(64), nullable=False, index=True),
+        sa.Column('platform', sa.String(50), nullable=False, index=True),
+        sa.Column('document_url', sa.String(2048), nullable=False),
+        sa.Column('document_name', sa.String(512), nullable=False),
+        sa.Column('parsed_rules', sa.JSON().with_variant(postgresql.JSONB, 'postgresql'), nullable=True),
+        sa.Column('status', sa.String(20), nullable=False, default='draft', index=True),
+        sa.Column('created_at', sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.Column('updated_at', sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table('platform_rules')