Add S3 presigned URL direct upload (presign → PUT → confirm)

- Add presign endpoint: POST /api/v1/transactions/presign
- Add confirm endpoint: POST /api/v1/transactions/confirm
- Add generate_presigned_put_url() to MinIO service with public endpoint support
- Add public_endpoint field to MinioConfig
- Keep legacy /upload endpoint as fallback

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

app/config.py

@@ -6,6 +6,7 @@ from pydantic import BaseModel
 
 class MinioConfig(BaseModel):
     endpoint: str
+    public_endpoint: str = ""
     bucket: str = "snapledger"
     access_key: str
     secret_key: str