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/src/db/seed-permissions.js

@@ -0,0 +1,4 @@
+'use strict';
+require('../controllers/permissionsController').seedDefaults();
+require('../controllers/gamificationController').seedAchievements();
+console.log('[seed-permissions] OK');