fix: extend username regex to allow hyphens, plus signs, and forward slashes

The regex pattern for the username field in UserModel was updated from
`^[a-zA-Z0-9_]+$` to `^[a-zA-Z0-9_-+/]+$`, adding support for hyphens,
plus signs, and forward slashes in usernames.

src/snek/model/user.py

@@ -8,7 +8,7 @@ class UserModel(BaseModel):
         required=True,
         min_length=2,
         max_length=20,
-        regex=r"^[a-zA-Z0-9_]+$",
+        regex=r"^[a-zA-Z0-9_-+/]+$",
     )
     nick = ModelField(
         name="nick",