Add pre-commit framework

- Install pre-commit
- Configure .pre-commit-config.yaml with:
  - ruff (lint + format) for Python
  - bandit for Python security
  - eslint for TypeScript
  - prettier for TypeScript
  - shared constants validation
- Add Makefile targets: pre-commit, lint

backend/pyproject.toml

@@ -23,6 +23,7 @@ dev = [
     "ruff>=0.14.10",
     "pytest-cov>=7.0.0",
     "bandit>=1.9.2",
+    "pre-commit>=4.5.1",
 ]
 
 [tool.mypy]