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/models/transaction.py

@@ -1,7 +1,18 @@
 from pydantic import BaseModel
 
 
-class UploadResponse(BaseModel):
-    success: bool
-    data: dict | None = None
-    error: str | None = None
+class PresignRequest(BaseModel):
+    original_filename: str
+
+
+class PresignData(BaseModel):
+    upload_url: str
+    object_key: str
+    ref_id: str
+
+
+class ConfirmRequest(BaseModel):
+    ref_id: str
+    object_key: str
+    original_filename: str
+    user_note: str | None = None