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>

routers/__init__.py

@@ -1,6 +1,8 @@
 from fastapi import APIRouter
 
 from .api import router as api_router
+from .wopi import wopi_router
 
 router = APIRouter()
-router.include_router(api_router)
+router.include_router(api_router)
+router.include_router(wopi_router)