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/queries/workspace.sql

@@ -16,7 +16,7 @@ SELECT a.id, a.generate_status, a.publish_status, a.source_type, a.created_at,
        t.template_name,
        COALESCE(
          NULLIF(gt.input_params_json ->> 'generation_mode', ''),
-         CASE WHEN a.source_type = 'custom_generation' THEN 'instant' ELSE NULL END
+         CASE WHEN a.source_type = 'custom_generation' THEN 'instant'::TEXT ELSE NULL::TEXT END
        ) AS generation_mode
 FROM articles a
 LEFT JOIN article_versions av ON av.id = a.current_version_id