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>

.gitignore

@@ -69,3 +69,13 @@ data/
 # JB 的运行配置（换设备用不了）
 .run/
 .xml
+
+# 前端构建产物（Docker 构建时复制）
+statics/
+
+# Cython 编译产物
+*.c
+
+# 许可证密钥（保密）
+license_private.pem
+license.key