GRM-20: refactor(scripts): centralize constants, API clients, and HTTP status codes

- Add shared config.py with API URLs, regexes, timeouts, pagination
- Add GiteaClient and VikunjaClient in api_clients.py with pooled sessions
- Add APIError exception for unified HTTP error handling
- Refactor all scripts to use shared modules and http.HTTPStatus
- Rewrite unit tests to mock clients and use HTTPStatus constants
- Add tests for api_clients and config modules
- Achieve 100% test coverage

src/gitea_runner_manager/exceptions.py

@@ -11,3 +11,12 @@ class AnsibleError(GRMError):
     """Raised when an Ansible command fails."""
 
     pass
+
+
+class APIError(GRMError):
+    """Raised when a REST API call returns an HTTP error."""
+
+    def __init__(self, status: int, message: str) -> None:
+        self.status = status
+        self.message = message
+        super().__init__(f"HTTP {status}: {message}")