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/src/config.ts

@@ -4,6 +4,11 @@ import {
   type IdentityConfig,
   type IdentityConfigInput,
 } from "@ekklesia/core/identity";
+import {
+  type ClientAddressConfig,
+  type ClientAddressConfigInput,
+  createClientAddressConfig,
+} from "./client-address.js";
 
 export type ApiConfigErrorCode =
   | "DATABASE_URL_INVALID"
@@ -37,6 +42,7 @@ const defaultHost = "127.0.0.1";
 const defaultPort = 3000;
 
 export interface ApiRuntimeConfig {
+  readonly clientAddress: ClientAddressConfig;
   readonly databaseUrl: string;
   readonly host: string;
   readonly identity: IdentityConfig;
@@ -44,6 +50,9 @@ export interface ApiRuntimeConfig {
 }
 
 export interface ApiRuntimeConfigInput {
+  /** Optional. Defaults to direct mode (trusts only the actual immediate socket peer) — no proxy
+   * trust unless explicitly configured. See `./client-address.js`. */
+  readonly clientAddress?: ClientAddressConfigInput;
   readonly databaseUrl?: unknown;
   readonly host?: unknown;
   readonly identity: IdentityConfigInput;
@@ -112,6 +121,7 @@ export function createApiRuntimeConfig(
     input.identity,
     options.environment === undefined ? {} : { environment: options.environment },
   );
+  const clientAddress = createClientAddressConfig(input.clientAddress);
 
-  return Object.freeze({ databaseUrl, host, identity, port });
+  return Object.freeze({ clientAddress, databaseUrl, host, identity, port });
 }