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/migrations/20260408110000_flatten_brand_questions.up.sql

@@ -0,0 +1,29 @@
+ALTER TABLE brand_questions
+    ADD COLUMN IF NOT EXISTS question_text TEXT;
+
+UPDATE brand_questions q
+SET question_text = COALESCE(
+    (
+        SELECT v.question_text
+        FROM brand_question_versions v
+        WHERE v.id = q.current_version_id
+        LIMIT 1
+    ),
+    (
+        SELECT v.question_text
+        FROM brand_question_versions v
+        WHERE v.question_id = q.id
+        ORDER BY v.version_no DESC, v.id DESC
+        LIMIT 1
+    ),
+    ''
+)
+WHERE q.question_text IS NULL;
+
+ALTER TABLE brand_questions
+    ALTER COLUMN question_text SET NOT NULL;
+
+ALTER TABLE brand_questions
+    DROP COLUMN IF EXISTS current_version_id;
+
+DROP TABLE IF EXISTS brand_question_versions;