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/theme_preset.py

@@ -3,9 +3,9 @@ from uuid import UUID
 
 from sqlmodel import Field
 
-from .base import SQLModelBase
+from sqlmodel_ext import SQLModelBase, UUIDTableBaseMixin
+
 from .color import ChromaticColor, NeutralColor, ThemeColorsBase
-from .mixin import UUIDTableBaseMixin
 
 
 class ThemePresetBase(SQLModelBase):