fix: secure authentication request identity

Replace Better Auth's default trust of a caller-controlled X-Forwarded-For
with a platform-owned client-address resolution boundary: direct mode (the
safe default) trusts only the real socket peer, and an explicit trusted-proxy
mode requires a header name, a concrete trusted-proxy list, and only honors a
forwarded chain from an actual trusted peer. This closes a rate-limit bypass
where rotating a spoofed header let a caller choose a fresh bucket on every
request. Public sign-up is also disabled by default and requires an explicit
typed opt-in, using Better Auth's own disableSignUp gate so no email-existence
leak or second signup path is introduced.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

apps/api/tests/module-database.integration.ts

@@ -263,6 +263,7 @@ async function startMigratedApi(
       host: "127.0.0.1",
       identity: {
         baseUrl: identityBaseUrl,
+        publicSignupEnabled: true,
         secret: "integration-test-secret-0123456789abcdefgh",
         trustedOrigins: [identityBaseUrl, dashboardOrigin],
       },