feat(admin-brand): add question expansion service and related functionality

- Implemented QuestionExpansionService for generating and materializing questions based on combinations and AI distillation.
- Added question metadata classification logic to infer intent and layer of questions.
- Created API handlers for question expansion operations including combination preview, AI distillation, metadata classification, and materialization.
- Introduced database migrations to support new question-related fields and constraints in brand_questions and brand_keywords tables.
- Added caching mechanism for AI distillation results to improve performance.
- Defined JSON schemas for question distillation responses to ensure data integrity.

server/internal/shared/config/config.go

@@ -237,10 +237,11 @@ func (p MembershipPlanConfig) QuotaPolicyJSON() ([]byte, error) {
 }
 
 type BrandLibraryConfig struct {
-	FreeBrandLimit         int `mapstructure:"free_brand_limit"`
-	PaidBrandLimit         int `mapstructure:"paid_brand_limit"`
-	MaxKeywords            int `mapstructure:"max_keywords"`
-	MaxQuestionsPerKeyword int `mapstructure:"max_questions_per_keyword"`
+	FreeBrandLimit         int            `mapstructure:"free_brand_limit"`
+	PaidBrandLimit         int            `mapstructure:"paid_brand_limit"`
+	MaxKeywords            int            `mapstructure:"max_keywords"`
+	MaxQuestionsPerKeyword int            `mapstructure:"max_questions_per_keyword"`
+	QuestionLimitsByPlan   map[string]int `mapstructure:"question_limits_by_plan"`
 }
 
 func (c BrandLibraryConfig) BrandLimitForPlan(planCode string) int {
@@ -250,6 +251,20 @@ func (c BrandLibraryConfig) BrandLimitForPlan(planCode string) int {
 	return c.PaidBrandLimit
 }
 
+func (c BrandLibraryConfig) QuestionLimitForPlan(planCode string) int {
+	limits := c.QuestionLimitsByPlan
+	normalizedPlan := strings.ToLower(strings.TrimSpace(planCode))
+	if limits != nil {
+		if value := limits[normalizedPlan]; value > 0 {
+			return value
+		}
+		if value := limits["default"]; value > 0 {
+			return value
+		}
+	}
+	return 25
+}
+
 type QdrantConfig struct {
 	URL        string        `mapstructure:"url"`
 	APIKey     string        `mapstructure:"api_key"`
@@ -1173,6 +1188,29 @@ func normalizeBrandLibraryConfig(cfg *BrandLibraryConfig) {
 	if cfg.MaxQuestionsPerKeyword <= 0 {
 		cfg.MaxQuestionsPerKeyword = 5
 	}
+	if cfg.QuestionLimitsByPlan == nil {
+		cfg.QuestionLimitsByPlan = map[string]int{}
+	}
+	normalized := make(map[string]int, len(cfg.QuestionLimitsByPlan)+4)
+	for key, value := range cfg.QuestionLimitsByPlan {
+		trimmed := strings.ToLower(strings.TrimSpace(key))
+		if trimmed != "" && value > 0 {
+			normalized[trimmed] = value
+		}
+	}
+	if normalized["default"] <= 0 {
+		normalized["default"] = 25
+	}
+	if normalized["free"] <= 0 {
+		normalized["free"] = 5
+	}
+	if normalized["plus"] <= 0 {
+		normalized["plus"] = 25
+	}
+	if normalized["pro"] <= 0 {
+		normalized["pro"] = 50
+	}
+	cfg.QuestionLimitsByPlan = normalized
 }
 
 func normalizeMembershipConfig(cfg *MembershipConfig) {

server/internal/shared/config/reload.go

@@ -62,7 +62,7 @@ func Diff(previous, current *Config) []FieldChange {
 	if !reflect.DeepEqual(previous.Membership, current.Membership) {
 		addChange("membership", true)
 	}
-	if previous.BrandLibrary != current.BrandLibrary {
+	if !reflect.DeepEqual(previous.BrandLibrary, current.BrandLibrary) {
 		addChange("brand_library", true)
 	}
 	if previous.Qdrant != current.Qdrant {