feat(kol): store KOL prompt content in database with object storage fallback

Add prompt_content columns to kol_prompts and kol_prompt_revisions so
prompt bodies live alongside their metadata, eliminating an extra round
trip to object storage for the common read path while keeping the old
asset key as a fallback for legacy rows.

Reads go through a singleflight-deduped, cache-backed loader that
prefers the database column, falls back to the asset key, and tolerates
missing objects via a new objectstorage.ErrObjectNotFound returned by
both the Aliyun OSS and MinIO clients on 404/NoSuchKey responses.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/internal/shared/objectstorage/minio.go

@@ -119,6 +119,9 @@ func (c *minioClient) GetBytes(ctx context.Context, objectKey string) ([]byte, e
 			zap.String("object_key", objectKey),
 			zap.Error(err),
 		)
+		if isMinIOObjectNotFound(err) {
+			return nil, fmt.Errorf("%w: %s", ErrObjectNotFound, objectKey)
+		}
 		return nil, fmt.Errorf("get minio object: %w", err)
 	}
 	defer reader.Close()
@@ -130,11 +133,21 @@ func (c *minioClient) GetBytes(ctx context.Context, objectKey string) ([]byte, e
 			zap.String("object_key", objectKey),
 			zap.Error(err),
 		)
+		if isMinIOObjectNotFound(err) {
+			return nil, fmt.Errorf("%w: %s", ErrObjectNotFound, objectKey)
+		}
 		return nil, fmt.Errorf("read minio object: %w", err)
 	}
 	return data, nil
 }
 
+func isMinIOObjectNotFound(err error) bool {
+	responseErr := minio.ToErrorResponse(err)
+	return responseErr.StatusCode == 404 ||
+		responseErr.Code == "NoSuchKey" ||
+		responseErr.Code == "NoSuchObject"
+}
+
 func (c *minioClient) Exists(ctx context.Context, objectKey string) (bool, error) {
 	if err := c.Validate(); err != nil {
 		return false, err