fix: add field_validator for users to handle single-string input

When a single user is passed via config/set query parameter, it arrives
as a plain string instead of a list, causing Pydantic validation to fail.
Add a validator matching the existing cameras pattern to wrap strings.

https://claude.ai/code/session_01PooiYnugPWqdCYDq4TU7ti

frigate/config/camera_group.py

@@ -36,3 +36,11 @@ class CameraGroupConfig(FrigateBaseModel):
             return [v]
 
         return v
+
+    @field_validator("users", mode="before")
+    @classmethod
+    def validate_users(cls, v):
+        if isinstance(v, str):
+            return [v]
+
+        return v