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/aliyun.go

@@ -141,6 +141,9 @@ func (c *aliyunClient) GetBytes(ctx context.Context, objectKey string) ([]byte,
 			zap.String("object_key", objectKey),
 			zap.Error(err),
 		)
+		if isAliyunObjectNotFound(err) {
+			return nil, fmt.Errorf("%w: %s", ErrObjectNotFound, objectKey)
+		}
 		return nil, fmt.Errorf("get aliyun object: %w", err)
 	}
 	defer reader.Close()
@@ -158,6 +161,16 @@ func (c *aliyunClient) GetBytes(ctx context.Context, objectKey string) ([]byte,
 	return data, nil
 }
 
+func isAliyunObjectNotFound(err error) bool {
+	serviceErr, ok := err.(oss.ServiceError)
+	if !ok {
+		return false
+	}
+	return serviceErr.StatusCode == 404 ||
+		serviceErr.Code == "NoSuchKey" ||
+		serviceErr.Code == "NoSuchObject"
+}
+
 func (c *aliyunClient) Exists(ctx context.Context, objectKey string) (bool, error) {
 	if err := c.Validate(); err != nil {
 		return false, err