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>

ee/__init__.py

@@ -0,0 +1,42 @@
+"""
+DiskNext Enterprise Edition (EE) 模块
+
+通过 ``try: from ee import init_ee`` 检测是否存在。
+CE 版本中此目录不存在，ImportError 被 main.py 捕获。
+"""
+from loguru import logger as l
+
+from utils.conf import appmeta
+
+_ee_initialized: bool = False
+
+
+def is_pro() -> bool:
+    """当前实例是否以 Pro 版本运行。"""
+    return _ee_initialized
+
+
+async def init_ee() -> None:
+    """
+    初始化企业版功能。
+
+    1. 加载并验证许可证
+    2. 设置 appmeta.IsPro = True
+    3. 标记 EE 已初始化
+
+    许可证无效或缺失时抛出异常，阻止应用启动。
+    """
+    global _ee_initialized
+
+    from ee.service.license_service import load_and_validate_license
+
+    payload = await load_and_validate_license()
+
+    appmeta.IsPro = True
+    _ee_initialized = True
+
+    l.info(
+        f"Pro 版本已激活 — 域名: {payload.domain}, "
+        f"过期: {payload.expires_at.isoformat()}, "
+        f"功能: {payload.features}"
+    )