Add src/auth.ts with Auth.js v5 config (task 3.1)

JWT session strategy, Credentials provider with bcryptjs password
verification, and Google OAuth provider using AUTH_GOOGLE_ID/SECRET.

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

src/auth.ts

@@ -0,0 +1,72 @@
+import NextAuth from "next-auth";
+import Credentials from "next-auth/providers/credentials";
+import Google from "next-auth/providers/google";
+import bcryptjs from "bcryptjs";
+import { eq } from "drizzle-orm";
+import { db } from "@/lib/db";
+import { users } from "@/lib/db/schema";
+
+export const { handlers, auth, signIn, signOut } = NextAuth({
+  secret: process.env.AUTH_SECRET,
+  session: {
+    strategy: "jwt",
+    maxAge: 30 * 24 * 60 * 60, // 30 days
+  },
+  providers: [
+    Credentials({
+      name: "Credentials",
+      credentials: {
+        email: { label: "Email", type: "email" },
+        password: { label: "Password", type: "password" },
+      },
+      async authorize(credentials) {
+        const email = credentials?.email as string | undefined;
+        const password = credentials?.password as string | undefined;
+
+        if (!email || !password) {
+          return null;
+        }
+
+        const [user] = await db
+          .select()
+          .from(users)
+          .where(eq(users.email, email))
+          .limit(1);
+
+        if (!user || !user.password_hash) {
+          return null;
+        }
+
+        const passwordValid = await bcryptjs.compare(password, user.password_hash);
+        if (!passwordValid) {
+          return null;
+        }
+
+        return {
+          id: user.id,
+          email: user.email,
+          name: user.name,
+          image: user.image,
+        };
+      },
+    }),
+    Google({
+      clientId: process.env.AUTH_GOOGLE_ID,
+      clientSecret: process.env.AUTH_GOOGLE_SECRET,
+    }),
+  ],
+  callbacks: {
+    async jwt({ token, user }) {
+      if (user) {
+        token.id = user.id;
+      }
+      return token;
+    },
+    async session({ session, token }) {
+      if (token.id) {
+        session.user.id = token.id as string;
+      }
+      return session;
+    },
+  },
+});