Harden form auth and submission handling

app/api/auth/[...nextauth]/route.ts

@@ -3,6 +3,7 @@ import DiscordProvider from "next-auth/providers/discord";
 import { DrizzleAdapter } from "@auth/drizzle-adapter";
 import { db } from "@/db";
 import { accounts, sessions, users, verificationTokens } from "@/db/schema";
+import type { SessionWithDiscord } from "@/lib/auth";
 
 const discordIdCache = new Map<string, string>();
 
@@ -22,7 +23,7 @@ export const authOptions: NextAuthOptions = {
   callbacks: {
     async session({ session, user }) {
       if (session.user) {
-        (session.user as any).id = user.id;
+        (session as SessionWithDiscord).user!.id = user.id;
 
         let discordId = discordIdCache.get(user.id);
         if (!discordId) {
@@ -37,7 +38,7 @@ export const authOptions: NextAuthOptions = {
           }
         }
         if (discordId) {
-          (session.user as typeof session.user & { discordId: string }).discordId = discordId;
+          (session as SessionWithDiscord).user!.discordId = discordId;
         }
       }
       return session;