Add pyproject.toml for consolidated project configuration and replace Black, isort, and Flake8 with Ruff

- Introduced `pyproject.toml` to centralize backend tool configurations (e.g., Ruff, mypy, coverage, pytest).
- Replaced Black, isort, and Flake8 with Ruff for linting, formatting, and import sorting.
- Updated `requirements.txt` to include Ruff and remove replaced tools.
- Added `Makefile` to streamline development workflows with commands for linting, formatting, type-checking, testing, and cleanup.

backend/requirements.txt

@@ -41,10 +41,8 @@ pytest-xdist>=3.8.0
 requests>=2.32.0
 
 # Development tools
-black>=24.3.0
-isort>=5.13.2
-flake8>=7.0.0
-mypy>=1.8.0
+ruff>=0.8.0  # All-in-one: linting, formatting, import sorting (replaces Black, Flake8, isort)
+mypy>=1.8.0  # Type checking
 
 # Security and authentication (pinned for reproducibility)
 python-jose==3.4.0