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/domain/brand_question.go

@@ -3,21 +3,11 @@ package domain
 import "time"
 
 type BrandQuestion struct {
-	ID               int64
-	TenantID         int64
-	BrandID          int64
-	KeywordID        int64
-	CurrentVersionID *int64
-	Status           string
-	CreatedAt        time.Time
-}
-
-type BrandQuestionVersion struct {
 	ID           int64
-	QuestionID   int64
+	TenantID     int64
+	BrandID      int64
+	KeywordID    int64
 	QuestionText string
-	QuestionHash string
-	VersionNo    int
-	IsActive     bool
+	Status       string
 	CreatedAt    time.Time
 }