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>

utils/http/http_exceptions.py

@@ -40,6 +40,10 @@ def raise_conflict(detail: str | None = None) -> NoReturn:
     """Raises an HTTP 409 Conflict exception."""
     raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail=detail)
 
+def raise_unprocessable_entity(detail: str | None = None) -> NoReturn:
+    """Raises an HTTP 422 Unprocessable Content exception."""
+    raise HTTPException(status_code=422, detail=detail)
+
 def raise_precondition_required(detail: str | None = None) -> NoReturn:
     """Raises an HTTP 428 Precondition required exception."""
     raise HTTPException(status_code=status.HTTP_428_PRECONDITION_REQUIRED, detail=detail)