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/services/minio.py

@@ -11,9 +11,9 @@ from app.config import settings
 def _get_client():
     return boto3.client(
         "s3",
-        endpoint_url=settings.minio.endpoint,
-        aws_access_key_id=settings.minio.access_key,
-        aws_secret_access_key=settings.minio.secret_key,
+        endpoint_url=settings.s3.endpoint,
+        aws_access_key_id=settings.s3.access_key,
+        aws_secret_access_key=settings.s3.secret_key,
         config=BotoConfig(signature_version="s3v4"),
     )
 
@@ -33,7 +33,7 @@ def upload_image(file_bytes: bytes, original_filename: str) -> str:
         content_type = "image/png"
 
     client.put_object(
-        Bucket=settings.minio.bucket,
+        Bucket=settings.s3.bucket,
         Key=object_key,
         Body=file_bytes,
         ContentType=content_type,
@@ -43,12 +43,12 @@ def upload_image(file_bytes: bytes, original_filename: str) -> str:
 
 def _get_presign_client():
     """Client using public endpoint so presigned URL signatures match the public host."""
-    endpoint = settings.minio.public_endpoint or settings.minio.endpoint
+    endpoint = settings.s3.public_endpoint or settings.s3.endpoint
     return boto3.client(
         "s3",
         endpoint_url=endpoint,
-        aws_access_key_id=settings.minio.access_key,
-        aws_secret_access_key=settings.minio.secret_key,
+        aws_access_key_id=settings.s3.access_key,
+        aws_secret_access_key=settings.s3.secret_key,
         config=BotoConfig(signature_version="s3v4"),
     )
 
@@ -58,7 +58,7 @@ def generate_presigned_put_url(object_key: str) -> str:
     url: str = client.generate_presigned_url(
         "put_object",
         Params={
-            "Bucket": settings.minio.bucket,
+            "Bucket": settings.s3.bucket,
             "Key": object_key,
         },
         ExpiresIn=300,