POC: Better Auth Google sign-in playground

[nhyiramante1]

What this is

A self-contained research spike in playgrounds/better-auth-test proving Better Auth
works in Next.js 15 before any production integration. No production code touched.

What was proven

Milestone 1 — Google OAuth sign-in ✅

• Better Auth + Google OAuth works in Next.js 15 App Router
• Session displays signed-in user email and name

Milestone 2A — Cookie-based protected endpoint ✅

• GET /api/protected returns 200 + user data with valid session cookie
• Returns 401 with no cookie

Milestone 2B — Bearer token protected endpoint ✅

• Same route accepts Authorization: Bearer <token> with no cookie
• Verified via browser network log with credentials: omit

Key config decisions

• serverExternalPackages: ["better-auth", "better-sqlite3"] required in next.config.ts
• Components using useSession() must use next/dynamic + ssr: false
• bearer() plugin enables cookie + Bearer on the same route handler

Not included

• No production writing-tools code changed
• No middleware-level auth
• Framework comparison (TanStack Start / React Router 7) is a future spike