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/services/minio.py

@@ -39,3 +39,23 @@ def upload_image(file_bytes: bytes, original_filename: str) -> str:
         ContentType=content_type,
     )
     return object_key
+
+
+def generate_presigned_put_url(object_key: str) -> str:
+    client = _get_client()
+    url: str = client.generate_presigned_url(
+        "put_object",
+        Params={
+            "Bucket": settings.minio.bucket,
+            "Key": object_key,
+        },
+        ExpiresIn=300,
+    )
+    # Replace internal endpoint with public endpoint
+    if settings.minio.public_endpoint:
+        from urllib.parse import urlparse, urlunparse
+
+        internal = urlparse(settings.minio.endpoint)
+        public = urlparse(settings.minio.public_endpoint)
+        url = url.replace(f"{internal.scheme}://{internal.netloc}", f"{public.scheme}://{public.netloc}", 1)
+    return url