feat: add comparison and user API endpoints

New API routes under src/app/api/ for comparisons and user operations.

src/app/api/user/stats/route.ts

@@ -0,0 +1,23 @@
+import { db } from "@/lib/db";
+import { comparisons } from "@/lib/db/schema";
+import { eq, sql } from "drizzle-orm";
+import { auth } from "@/lib/auth";
+import { headers } from "next/headers";
+
+export async function GET() {
+  const session = await auth.api.getSession({ headers: await headers() });
+
+  if (!session?.user) {
+    return Response.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  const result = await db
+    .select({
+      totalComparisons: sql<number>`count(*)`,
+      totalViews: sql<number>`coalesce(sum(${comparisons.viewCount}), 0)`,
+    })
+    .from(comparisons)
+    .where(eq(comparisons.userId, session.user.id));
+
+  return Response.json(result[0]);
+}