extend POST /users/:id

Now you can set a password for a user while the server is running,
e.g. via the following command:

```shell
curl \
    -H 'Content-Type: application/json' \
    -d '{"update": {"password": "asdf"}}' \
    --unix-socket /var/lib/moonfire-nvr/sock \
    http://nvr/api/users/1
```

server/db/db.rs

@@ -2034,6 +2034,10 @@ impl LockedDatabase {
         self.auth.users_by_id()
     }
 
+    pub fn get_user_by_id_mut(&mut self, id: i32) -> Option<&mut User> {
+        self.auth.get_user_by_id_mut(id)
+    }
+
     pub fn apply_user_change(&mut self, change: UserChange) -> Result<&User, Error> {
         self.auth.apply(&self.conn, change)
     }