feat(auth): add WorkOS + Keycloak SSO providers (P8-001)

- Refactor auth.ts to build OAuth providers array dynamically; extract
  buildOAuthProviders() for unit-testability
- Add WorkOS provider (WORKOS_CLIENT_ID/SECRET/REDIRECT_URI env vars)
- Add Keycloak provider with realm-scoped OIDC discovery
  (KEYCLOAK_URL/REALM/CLIENT_ID/CLIENT_SECRET env vars)
- Add genericOAuthClient plugin to web auth-client for signIn.oauth2()
- Add WorkOS + Keycloak SSO buttons to login page (NEXT_PUBLIC_*_ENABLED
  feature flags control visibility)
- Update .env.example with SSO provider stanzas
- Add 8 unit tests covering all provider inclusion/exclusion paths

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/lib/auth-client.ts

@@ -1,9 +1,9 @@
 import { createAuthClient } from 'better-auth/react';
-import { adminClient } from 'better-auth/client/plugins';
+import { adminClient, genericOAuthClient } from 'better-auth/client/plugins';
 
 export const authClient = createAuthClient({
   baseURL: process.env['NEXT_PUBLIC_GATEWAY_URL'] ?? 'http://localhost:4000',
-  plugins: [adminClient()],
+  plugins: [adminClient(), genericOAuthClient()],
 });
 
 export const { useSession, signIn, signUp, signOut } = authClient;