feat(dx): pre-commit hooks for local CI (no remote needed)

Local quality gate that runs on every git commit:
- node --check syntax on staged .js files
- block on new emoji in staged .js/.html/.css (md files allowed)
- block on new console.log/debug/debugger statements
- backend route auth lint (existing npm run lint:routes)
- backend tests (baseline 3 fails, block if grew)

Install: npm run hooks:install (top-level)
Bypass: git commit --no-verify

Skips slow checks when irrelevant files changed (tests/lint only run
if backend touched).

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

backend/package.json

@@ -13,7 +13,8 @@
     "seed:permissions": "node src/db/seed-permissions.js",
     "lint:routes": "node scripts/check-route-auth.js",
     "import:content": "node scripts/import-content.js",
-    "test": "node --test tests/*.test.js"
+    "test": "node --test tests/*.test.js",
+    "hooks:install": "sh ../scripts/install-hooks.sh"
   },
   "dependencies": {
     "bcryptjs": "^2.4.3",