Fix WebDAV 500 when htpasswd file doesn't exist yet

Create an empty /etc/micronas-webdav.passwd on first webdav config write
so Apache returns 401 instead of 500 before any user password is set

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -306,6 +306,9 @@ def _write_webdav_conf(shares):
         ]
     with open(WEBDAV_CONF, 'w') as f:
         f.writelines(lines)
+    # Ensure the passwd file exists so Apache doesn't 500 before any user has logged in
+    if not os.path.exists(WEBDAV_PASSWD):
+        Path(WEBDAV_PASSWD).touch(mode=0o640)
     subprocess.run(['a2enconf', 'micronas-webdav'], capture_output=True)
     subprocess.run(['systemctl', 'reload', 'apache2'], capture_output=True)