🦺(migration) add back the migration folders to linter

Previous commit add "core/tests/migrations".
The linter could not pass on it because all the
migration folders were excluded from the linter.
We remove this exclusion, tests and migrations can
now be linted and formatted automatically.

src/backend/pyproject.toml

@@ -100,7 +100,6 @@ exclude = [
     "build",
     "venv",
     "__pycache__",
-    "*/migrations/*",
 ]
 line-length = 88