Remove Apple SSO, keep Google + GitHub only

- Removed Apple from OAuth lib (config, exchange, provider type)
- Removed Apple button from auth page
- Removed Apple tab from setup guide page
- Removed Apple env vars from .env, .env.example, docker-compose.yml
- Removed Apple prompts from setup-sso.sh

src/app/api/auth/[provider]/route.ts

@@ -7,7 +7,7 @@ import {
   buildAuthorizationUrl,
 } from "@/lib/oauth";
 
-const VALID_PROVIDERS = ["google", "apple", "github"];
+const VALID_PROVIDERS = ["google", "github"];
 
 export async function GET(
   _req: NextRequest,