Fix WebDAV startup with WsgiDAV 4.x middleware stack

[TheZupZup]

Motivation

• The WebDAV server failed to start with a ModuleNotFoundError because wsgidav.debug_filter.WsgiDavDebugFilter is not present in some WsgiDAV 4.x installs, causing startup to crash.

Description

• Remove the unconditional wsgidav.debug_filter.WsgiDavDebugFilter entry from the middleware_stack in nexanote/sync/server.py and import importlib.util.
• Add a runtime check using importlib.util.find_spec("wsgidav.debug_filter") to prepend the debug middleware only when the module is available, preserving debug support when installed.
• This change keeps the WebDAV startup compatible across WsgiDAV versions while retaining debug middleware if present.

Testing

• Ran timeout 8s python main.py and verified the backend starts without the previous ModuleNotFoundError (startup completed within the timeout).
• Ran pytest -q and observed 64 passed (all tests succeeded).

---

Codex Task