docs(server): restore Better Auth integration guide for v2

apps/content/docs/integrations/better-auth.mdx

@@ -0,0 +1,112 @@
+---
+title: "Better Auth Integration"
+description: "Use Better Auth sessions in oRPC context and protect procedures with typed middleware."
+sidebar:
+  label: "Better Auth"
+---
+
+Use your [Better Auth](https://better-auth.com/) instance with oRPC's [context](/docs/context) and [middleware](/docs/middleware). No extra package is needed.
+
+:::tip
+You may need to forward Better Auth's [response headers](https://better-auth.com/docs/concepts/api#getting-headers), such as a refreshed session cookie. The [Response Headers Plugin](/docs/plugins/response-headers) can help unless you use the [Batch Plugin](/docs/plugins/batch).
+:::
+
+## Resolve the Session in Middleware
+
+The [Request Headers Plugin](/docs/plugins/request-headers) exposes request headers as `context.reqHeaders`. The middleware loads the session from them and rejects unauthenticated calls. Public procedures use the base directly. Each protected call performs its own lookup, including every sub-request of a [batch](/docs/plugins/batch).
+
+```ts
+import type { RequestHeadersHandlerPluginContext } from '@orpc/server/plugins'
+import { ORPCError, os } from '@orpc/server'
+
+interface ServerContext extends RequestHeadersHandlerPluginContext {}
+
+const base = os.$context<ServerContext>()
+
+const requireSession = base.middleware(async ({ context, next }) => {
+  const session = await auth.api.getSession({
+    headers: context.reqHeaders ?? new Headers(),
+  })
+
+  if (!session) {
+    throw new ORPCError('UNAUTHORIZED')
+  }
+
+  return next({ context: { session } })
+})
+
+const protectedProcedure = base.use(requireSession)
+
+const router = {
+  ping: base.handler(() => ({ message: 'pong' })),
+  me: protectedProcedure.handler(({ context }) => ({
+    id: context.session.user.id,
+    name: context.session.user.name,
+  })),
+}
+```
+
+`context.session` is Better Auth's full result with `user` and `session`. Its type is inferred from your auth instance, so additional fields stay available.
+
+Only a missing session becomes `UNAUTHORIZED`. Other errors from `getSession` propagate to oRPC's [error handling](/docs/error-handling).
+
+`reqHeaders` is `undefined` without the plugin, such as in [server-side calls](/docs/client/server-side). The empty `Headers` fallback carries no session cookie, so `getSession` returns `null` and protected calls return `UNAUTHORIZED`. Pass `reqHeaders` in the initial context to authenticate such calls.
+
+## Lazily Load and Share the Session
+
+If your server resolves the session itself, pass a lazy getter into the initial context instead of the session. The lookup runs at most once per request and only when a procedure asks for it. This includes [batch](/docs/plugins/batch) requests, where every sub-request shares the getter. The same getter can also serve the rest of your request handling.
+
+```ts
+import { ORPCError, os } from '@orpc/server'
+import { RPCHandler } from '@orpc/server/fetch'
+
+type Session = Awaited<ReturnType<typeof auth.api.getSession>>
+
+function once<T>(fn: () => Promise<T>): () => Promise<T> {
+  let promise: Promise<T> | undefined
+
+  return () => {
+    promise ??= fn()
+    return promise
+  }
+}
+
+const base = os.$context<{ getSession: () => Promise<Session> }>()
+
+const requireSession = base.middleware(async ({ context, next }) => {
+  const session = await context.getSession()
+
+  if (!session) {
+    throw new ORPCError('UNAUTHORIZED')
+  }
+
+  return next({ context: { session } })
+})
+
+const protectedProcedure = base.use(requireSession)
+
+const router = {
+  greeting: base.handler(async ({ context }) => {
+    const session = await context.getSession()
+
+    return { message: `Hello, ${session?.user.name ?? 'guest'}` }
+  }),
+  me: protectedProcedure.handler(({ context }) => ({
+    id: context.session.user.id,
+    name: context.session.user.name,
+  })),
+}
+
+const handler = new RPCHandler(router)
+
+export async function fetch(request: Request): Promise<Response> {
+  const getSession = once(() => auth.api.getSession({ headers: request.headers }))
+
+  const { matched, response } = await handler.handle(request, {
+    prefix: '/rpc',
+    context: { getSession },
+  })
+
+  return matched ? response : new Response('Not Found', { status: 404 })
+}
+```