feat: S3-compatible object storage backend

BACKEND_STORAGE_DRIVER=s3 swaps LocalStore for a stdlib-only SigV4
implementation — PutObject/GetObject/DeleteObject/CopyObject/ListObjectsV2
against any S3-compatible API (AWS, MinIO, Garage, R2). Storage becomes a
Store interface; storage_path records s3://bucket/key. Content-Length is
resolved by seeking or temp-file spooling — chunked uploads are rejected
by real S3 servers. Bucket auto-created on startup.

Verified live against zenko/cloudserver: bucket, put, get, copy, delete.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/storage/local.go

@@ -65,7 +65,7 @@ func (s *LocalStore) Put(ctx context.Context, bucketID, objectKey string, reader
 	}, nil
 }
 
-func (s *LocalStore) Open(bucketID, objectKey string) (*os.File, string, error) {
+func (s *LocalStore) Open(bucketID, objectKey string) (io.ReadCloser, string, error) {
 	cleanKey, err := sanitizeObjectKey(objectKey)
 	if err != nil {
 		return nil, "", err