revert: remove sensitive values from .env.example

Author: pengyu

backend/.env.example

@@ -13,13 +13,13 @@ OPENAI_BASE_URI="http://localhost:3001"
 
 # S3/Cloudflare R2 Configuration (Optional)
 # If not provided, local file storage will be used
-S3_ACCESS_KEY_ID="6215aeddb7631fc0d6284ddbb63364f8"      # Must be 32 characters for Cloudflare R2
-S3_SECRET_ACCESS_KEY="66c6c645f27e72b6bbb1de479163eb454cd4aaee212f5767a660525cb4bd813a"
+S3_ACCESS_KEY_ID="your_s3_access_key_id"      # Must be 32 characters for Cloudflare R2
+S3_SECRET_ACCESS_KEY="your_s3_secret_access_key"
 S3_REGION="auto"                              # Use 'auto' for Cloudflare R2
-S3_BUCKET_NAME="pengyucdn"
-S3_ENDPOINT="https://a85330980c7cb2d6526f81850a64fced.r2.cloudflarestorage.com"    # Cloudflare R2 endpoint
-S3_ACCOUNT_ID="a85330980c7cb2d6526f81850a64fced"   # Your Cloudflare account ID
-S3_PUBLIC_URL="https://pub-47e72e8a290d495a81fe0741fd1a2f1a.r2.dev"        # Your R2 public bucket URL
+S3_BUCKET_NAME="your_bucket_name"
+S3_ENDPOINT="https://<account_id>.r2.cloudflarestorage.com"    # Cloudflare R2 endpoint
+S3_ACCOUNT_ID="your_cloudflare_account_id"    # Your Cloudflare account ID
+S3_PUBLIC_URL="https://pub-xxx.r2.dev"        # Your R2 public bucket URL
 
 # mail
 # Set to false to disable all email functionality