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>

app/config.py

@@ -4,7 +4,7 @@ import yaml
 from pydantic import BaseModel
 
 
-class MinioConfig(BaseModel):
+class S3Config(BaseModel):
     endpoint: str
     public_endpoint: str = ""
     bucket: str = "snapledger"
@@ -32,7 +32,7 @@ class ApiConfig(BaseModel):
 
 
 class AppConfig(BaseModel):
-    minio: MinioConfig
+    s3: S3Config
     postgresql: PostgresConfig
     api: ApiConfig