feat: add signin API and setup route

src/app/api/auth/signin/route.ts

@@ -0,0 +1,33 @@
+import { NextResponse } from "next/server";
+import { db } from "@/db";
+import { users } from "@/db/schema/auth";
+import { eq } from "drizzle-orm";
+
+export async function POST(request: Request) {
+  const { email } = await request.json();
+
+  if (!email) {
+    return NextResponse.json({ error: "Email required" }, { status: 400 });
+  }
+
+  try {
+    // Find or create user
+    let user = await db.query.users.findFirst({
+      where: eq(users.email, email),
+    });
+
+    if (!user) {
+      const [newUser] = await db
+        .insert(users)
+        .values({ email })
+        .returning();
+      user = newUser;
+    }
+
+    // For demo, just return success - in real app, send magic link via email
+    return NextResponse.json({ success: true, userId: user.id });
+  } catch (error) {
+    console.error(error);
+    return NextResponse.json({ error: String(error) }, { status: 500 });
+  }
+}