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

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

packages/db/prisma/migrations/20260502000003_add_ai_settings/migration.sql

@@ -0,0 +1,10 @@
+CREATE TABLE "ai_settings" (
+    "id" SERIAL NOT NULL,
+    "userId" INTEGER NOT NULL,
+    "apiKey" TEXT NOT NULL,
+    CONSTRAINT "ai_settings_pkey" PRIMARY KEY ("id")
+);
+
+ALTER TABLE "ai_settings" ADD CONSTRAINT "ai_settings_userId_fkey" FOREIGN KEY ("userId") REFERENCES "User"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+CREATE UNIQUE INDEX "ai_settings_userId_key" ON "ai_settings"("userId");