feat: add tests for import polling, login page, money utilities, and query keys

- Implemented tests for import polling functionality, ensuring correct behavior for active and terminal import statuses.
- Created tests for the login page, validating user interactions and error handling.
- Added unit tests for money utility functions, covering parsing, formatting, and summation of monetary values.
- Developed tests for query keys, ensuring proper scoping and invalidation roots.

chore: setup testing environment and configuration

- Added setup file for testing with Vitest and Testing Library.
- Configured TypeScript and Vite for the frontend project.
- Created Docker and Nginx configurations for local development and production environments.
- Established a PostgreSQL initialization script for setting up application roles.

docs: add README and environment configuration for infra

- Included README for the infra directory, detailing local development setup and service descriptions.
- Created an example environment configuration file for local development with placeholders for sensitive information.

backend/app/errors.py

@@ -0,0 +1,48 @@
+from typing import Any
+
+
+class AppError(Exception):
+    """Domain error rendered as a machine-readable JSON error payload."""
+
+    status_code: int = 400
+    code: str = "APP_ERROR"
+
+    def __init__(
+        self,
+        message: str,
+        *,
+        code: str | None = None,
+        status_code: int | None = None,
+        details: Any = None,
+    ) -> None:
+        super().__init__(message)
+        self.message = message
+        if code is not None:
+            self.code = code
+        if status_code is not None:
+            self.status_code = status_code
+        self.details = details
+
+
+def bad_request(code: str, message: str, details: Any = None) -> AppError:
+    return AppError(message, code=code, status_code=400, details=details)
+
+
+def unauthorized(code: str, message: str, details: Any = None) -> AppError:
+    return AppError(message, code=code, status_code=401, details=details)
+
+
+def forbidden(code: str, message: str, details: Any = None) -> AppError:
+    return AppError(message, code=code, status_code=403, details=details)
+
+
+def not_found(code: str, message: str, details: Any = None) -> AppError:
+    return AppError(message, code=code, status_code=404, details=details)
+
+
+def conflict(code: str, message: str, details: Any = None) -> AppError:
+    return AppError(message, code=code, status_code=409, details=details)
+
+
+def unprocessable(code: str, message: str, details: Any = None) -> AppError:
+    return AppError(message, code=code, status_code=422, details=details)