fix: use UUID for Better Auth ID generation to match Prisma schema

Better Auth generates nanoid-style IDs by default, but our Prisma
schema uses @db.Uuid columns for all auth tables. This caused
P2023 errors when Better Auth tried to insert non-UUID IDs into
the verification table during OAuth sign-in.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

apps/api/src/auth/auth.config.ts

@@ -1,3 +1,4 @@
+import { randomUUID } from "node:crypto";
 import { betterAuth } from "better-auth";
 import { prismaAdapter } from "better-auth/adapters/prisma";
 import { genericOAuth } from "better-auth/plugins";
@@ -216,6 +217,7 @@ export function createAuth(prisma: PrismaClient) {
       updateAge: 60 * 60 * 2, // 2 hours — minimum session age before BetterAuth refreshes the expiry on next request
     },
     advanced: {
+      generateId: () => randomUUID(),
       defaultCookieAttributes: {
         httpOnly: true,
         secure: process.env.NODE_ENV === "production",