feat(auth): Configure Authentik OIDC integration with better-auth

- Add genericOAuth plugin to auth.config.ts with Authentik provider
- Fix LoginButton to use /auth/signin/authentik (not /auth/callback/)
- Add production URLs to trustedOrigins
- Update .env.example with correct redirect URI documentation

Redirect URI for Authentik: https://api.mosaicstack.dev/auth/callback/authentik

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/src/components/auth/LoginButton.tsx

@@ -8,7 +8,7 @@ export function LoginButton(): React.JSX.Element {
   const handleLogin = (): void => {
     // Redirect to the backend OIDC authentication endpoint
     // BetterAuth will handle the OIDC flow and redirect back to the callback
-    window.location.assign(`${API_URL}/auth/callback/authentik`);
+    window.location.assign(`${API_URL}/auth/signin/authentik`);
   };
 
   return (