Add pre-commit hooks for linting and formatting

Set up pre-commit framework with ruff (backend) and ESLint/Prettier/tsc
(frontend) hooks to catch issues locally before CI. Auto-format all
frontend files with Prettier to comply with the new check.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/pyproject.toml

@@ -18,6 +18,7 @@ dependencies = [
 [project.optional-dependencies]
 dev = [
     "ruff>=0.9.0",
+    "pre-commit>=4.0.0",
     "pytest>=8.0.0",
     "pytest-asyncio>=0.25.0",
     "httpx>=0.28.0",