feat: migrate ORM base to sqlmodel-ext, add file viewers and WOPI integration

- Migrate SQLModel base classes, mixins, and database management to
  external sqlmodel-ext package; remove sqlmodels/base/, sqlmodels/mixin/,
  and sqlmodels/database.py
- Add file viewer/editor system with WOPI protocol support for
  collaborative editing (OnlyOffice, Collabora)
- Add enterprise edition license verification module (ee/)
- Add Dockerfile multi-stage build with Cython compilation support
- Add new dependencies: sqlmodel-ext, cryptography, whatthepatch

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

ee/service/__init__.py

@@ -0,0 +1,30 @@
+"""
+EE 许可证服务模块
+
+提供许可证加载、验证和缓存功能。
+"""
+from datetime import datetime
+
+from sqlmodel_ext import SQLModelBase
+
+
+class LicensePayload(SQLModelBase):
+    """许可证载荷（RSA 验签后的明文数据）"""
+
+    domain: str
+    """授权域名"""
+
+    expires_at: datetime
+    """过期时间（UTC）"""
+
+    max_users: int
+    """最大用户数（0 = 无限制）"""
+
+    features: list[str]
+    """已授权的功能列表"""
+
+    issued_at: datetime
+    """签发时间（UTC）"""
+
+
+from .license_service import get_cached_license, load_and_validate_license