Fix: Samba config PUT 422 + shadow:format setparm dash escaping

Route /samba/{name} was matching before /samba/config, causing 422 on
PUT /samba/config. Moved static routes before parameterized routes.

Values starting with '-' (shadow: format = -%Y-%m-%d-%H%M) were parsed
as CLI flags by net conf setparm. Added '--' separator to escape them.

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

backend/services/shares.py

@@ -255,7 +255,7 @@ class SharesManager:
         try:
             for key, value in parameters.items():
                 result = subprocess.run(
-                    ['/usr/bin/net', 'conf', 'setparm', 'global', key, value],
+                    ['/usr/bin/net', 'conf', 'setparm', 'global', key, '--', value],
                     capture_output=True,
                     text=True,
                     timeout=10