fix: resolve all 28 mypy type errors + re-enable coverage in CI

backend/auth/router.py

@@ -250,6 +250,7 @@ async def change_password(
 ):
     """Change own password."""
     user = get_user(current_user["username"])
+    assert user is not None, f"User {current_user['username']} not found"
     if not verify_password(req.current_password, user["password_hash"]):
         raise HTTPException(400, "Mot de passe actuel incorrect")
     update_user(current_user["username"], {"password": req.new_password})