fix: export guard dependencies so cross-module @UseGuards resolves

NestJS resolves a guard referenced via @UseGuards(SomeGuard) using the
consuming module's injector, not the guard's own declaring module's
injector. OidcAuthGuard and TripMembershipGuard are shared across
several feature modules, so every constructor dependency they need
(UsersRepository/UsersService, TripMembersRepository, etc.) must be
re-exported by AuthModule/TripsLibModule/UsersLibModule, not just the
guard classes themselves. Found via the Phase 02 end-to-end smoke test
against a mocked IdP, which failed to boot the API before this fix.

backend/libs/auth/src/auth.module.ts

@@ -1,11 +1,12 @@
-import { Module } from '@nestjs/common';
+import { Global, Module } from '@nestjs/common';
 import { UsersLibModule } from '../../users/src';
 import { OidcDiscoveryService } from './oidc-discovery.service';
 import { OidcAuthGuard } from './oidc-auth.guard';
 
+@Global()
 @Module({
   imports: [UsersLibModule],
   providers: [OidcDiscoveryService, OidcAuthGuard],
-  exports: [OidcDiscoveryService, OidcAuthGuard],
+  exports: [OidcDiscoveryService, OidcAuthGuard, UsersLibModule],
 })
 export class AuthModule {}