feat: add AI settings routes, storage, UI card, and migration

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

apps/Backend/src/storage/ai-settings-storage.ts

@@ -0,0 +1,15 @@
+import { prisma as db } from "@repo/db/client";
+
+export const aiSettingsStorage = {
+  async getAiSettings(userId: number) {
+    return db.aiSettings.findUnique({ where: { userId } });
+  },
+
+  async upsertAiSettings(userId: number, apiKey: string) {
+    return db.aiSettings.upsert({
+      where: { userId },
+      update: { apiKey },
+      create: { userId, apiKey },
+    });
+  },
+};