refactor: explicit migrate/seed scripts, no auto-init at server start

Auto-running migrations on every server boot is dangerous — a broken
migration silently corrupts data or blocks server start. Now require
explicit `npm run migrate && npm run seed:permissions` before start.
Boot asserts schema exists (users + role_permissions tables) and
fails fast with a clear message otherwise.

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

backend/package.json

@@ -8,6 +8,7 @@
     "dev": "nodemon src/server.js",
     "migrate": "node src/db/migrate.js",
     "seed": "node src/db/seed.js",
+    "seed:permissions": "node src/db/seed-permissions.js",
     "test": "node --test tests/*.test.js"
   },
   "dependencies": {