backend: per-user settings store (/api/settings) + enable account deletion

- user_settings table (userId PK -> user, jsonb preferences) + migration
- GET/PUT /api/settings (requireAuth, user-scoped; zod-validated flat map)
- Better Auth user.deleteUser enabled so users can delete their own account

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/db/schema/index.ts

@@ -7,3 +7,4 @@ export * from "./tasks.js";
 export * from "./activity.js";
 export * from "./messaging.js";
 export * from "./notifications.js";
+export * from "./settings.js";