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>

sqlmodels/user.py

@@ -9,11 +9,11 @@ from sqlmodel import Field, Relationship
 from sqlmodel.ext.asyncio.session import AsyncSession
 from sqlmodel.main import RelationshipInfo
 
+from sqlmodel_ext import SQLModelBase, UUIDTableBaseMixin, TableViewRequest, ListResponse
+
 from .auth_identity import AuthProviderType
-from .base import SQLModelBase
 from .color import ChromaticColor, NeutralColor, ThemeColorsBase
 from .model_base import ResponseBase
-from .mixin import UUIDTableBaseMixin, TableViewRequest, ListResponse
 
 T = TypeVar("T", bound="User")