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/webdav.py

@@ -4,8 +4,7 @@ from uuid import UUID
 
 from sqlmodel import Field, Relationship, UniqueConstraint
 
-from .base import SQLModelBase
-from .mixin import TableBaseMixin
+from sqlmodel_ext import SQLModelBase, TableBaseMixin
 
 if TYPE_CHECKING:
     from .user import User