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";

backend/src/db/schema/settings.ts

@@ -0,0 +1,20 @@
+import { jsonb, pgTable, text, timestamp } from "drizzle-orm/pg-core";
+
+import { user } from "./auth.js";
+
+// Per-user preferences (notification toggles, profile extras). One row per
+// user, keyed by the Better Auth user id; the payload is a flat JSONB map so
+// the frontend can add settings without a migration.
+export const userSettings = pgTable("user_settings", {
+  userId: text("user_id")
+    .primaryKey()
+    .references(() => user.id, { onDelete: "cascade" }),
+  preferences: jsonb("preferences")
+    .$type<Record<string, boolean | string>>()
+    .notNull()
+    .default({}),
+  updatedAt: timestamp("updated_at")
+    .defaultNow()
+    .$onUpdate(() => new Date())
+    .notNull(),
+});