Rename MinIO config to S3, switch to Cloudflare R2

- Rename MinioConfig → S3Config, settings.minio → settings.s3
- Update config.example.yaml with R2 endpoint template
- S3 service is now storage-backend agnostic (MinIO, R2, any S3-compatible)

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

config.example.yaml

@@ -1,12 +1,12 @@
 # SnapLedger Backend Configuration
 # Copy this file to config.yaml and fill in your credentials.
 
-minio:
-  endpoint: "https://minio.example.com:9000"
-  public_endpoint: "https://minio-public.example.com"
+s3:
+  endpoint: "https://<account-id>.r2.cloudflarestorage.com"
+  public_endpoint: ""
   bucket: "snapledger"
-  access_key: "YOUR_MINIO_ACCESS_KEY"
-  secret_key: "YOUR_MINIO_SECRET_KEY"
+  access_key: "YOUR_S3_ACCESS_KEY"
+  secret_key: "YOUR_S3_SECRET_KEY"
 
 postgresql:
   host: "localhost"