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

.pre-commit-config.yaml

@@ -0,0 +1,41 @@
+repos:
+  - repo: https://github.com/astral-sh/ruff-pre-commit
+    rev: v0.14.0
+    hooks:
+      - id: ruff
+        args: [--fix]
+        files: ^backend/
+      - id: ruff-format
+        files: ^backend/
+
+  - repo: https://github.com/PyCQA/bandit
+    rev: 1.8.0
+    hooks:
+      - id: bandit
+        args: [-c, backend/pyproject.toml, -r]
+        files: ^backend/
+        exclude: ^backend/(tests|\.venv)/
+
+  - repo: local
+    hooks:
+      - id: eslint
+        name: ESLint
+        entry: npm run lint --prefix frontend
+        language: system
+        files: ^frontend/.*\.(ts|tsx)$
+        pass_filenames: false
+
+      - id: prettier
+        name: Prettier
+        entry: npm run format:check --prefix frontend
+        language: system
+        files: ^frontend/
+        pass_filenames: false
+
+      - id: check-constants
+        name: Check shared constants
+        entry: bash -c 'cd backend && uv run python validate_constants.py'
+        language: system
+        files: shared/constants\.json$
+        pass_filenames: false
+