feat: Introduce AI Brand Monitoring System V5 technical design document

- Added comprehensive technical design document for AI Brand Monitoring System V5, outlining system architecture, data models, sampling strategies, and monitoring protocols.
- Key changes include a shift to a sampling-based trend monitoring approach, updated data collection and storage strategies, and new metrics for performance evaluation.
- Implemented migration scripts to support the flattening of brand questions and versioning of question texts, ensuring historical data integrity and version control.

server/internal/tenant/repository/common.go

@@ -20,6 +20,22 @@ func nullableText(value pgtype.Text) *string {
 	return &text
 }
 
+func nullableAnyText(value interface{}) *string {
+	switch typed := value.(type) {
+	case nil:
+		return nil
+	case string:
+		return &typed
+	case []byte:
+		text := string(typed)
+		return &text
+	case pgtype.Text:
+		return nullableText(typed)
+	default:
+		return nil
+	}
+}
+
 func nullableInt64(value pgtype.Int8) *int64 {
 	if !value.Valid {
 		return nil