fix(brand-questions): surface seed topic min length as actionable error

Reject AI seed topics shorter than 2 characters on both client and server with
a dedicated invalid_seed_topic error code, and render an inline hint listing
the offending terms so users know exactly what to fix.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/internal/tenant/app/question_expansion_service.go

@@ -218,7 +218,7 @@ func (s *QuestionExpansionService) GenerateByAIDistill(ctx context.Context, bran
 	actor := auth.MustActor(ctx)
 	seedTopic := normalizeQuestionText(req.SeedTopic)
 	if utf8.RuneCountInString(seedTopic) < 2 {
-		return nil, response.ErrBadRequest(40091, "invalid_params", "seed_topic is required")
+		return nil, response.ErrBadRequest(40091, "invalid_seed_topic", "seed_topic must contain at least 2 characters")
 	}
 
 	brandCtx, err := s.loadQuestionBrandContext(ctx, actor.TenantID, brandID)

server/internal/tenant/transport/question_expansion_handler.go

@@ -2,6 +2,7 @@ package transport
 
 import (
 	"strconv"
+	"strings"
 
 	"github.com/gin-gonic/gin"
 
@@ -61,6 +62,10 @@ func (h *QuestionExpansionHandler) AIDistill(c *gin.Context) {
 	}
 	var req app.QuestionDistillRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
+		if isInvalidSeedTopicBindError(err) {
+			response.Error(c, response.ErrBadRequest(40091, "invalid_seed_topic", "seed_topic must contain at least 2 characters"))
+			return
+		}
 		response.Error(c, response.ErrBadRequest(40001, "invalid_params", err.Error()))
 		return
 	}
@@ -118,3 +123,12 @@ func parseBrandIDParam(c *gin.Context) (int64, bool) {
 	}
 	return brandID, true
 }
+
+func isInvalidSeedTopicBindError(err error) bool {
+	if err == nil {
+		return false
+	}
+	message := err.Error()
+	return strings.Contains(message, "QuestionDistillRequest.SeedTopic") ||
+		strings.Contains(message, "seed_topic")
+}