feat: add PATCH /tags/{id}/rules/{then_id} to activate/deactivate rules

- openapi.yaml: new PATCH endpoint with is_active body, returns TagRule
- backend/service: SetRuleActive calls repo.SetActive then returns updated rule
- backend/handler: PatchRule validates body and delegates to service
- backend/router: register PATCH /:tag_id/rules/:then_tag_id
- frontend: TagRuleEditor uses PATCH instead of delete+recreate
- mock: handle PATCH /tags/{id}/rules/{then_id}

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/handler/router.go

@@ -93,6 +93,7 @@ func NewRouter(
 
 		tags.GET("/:tag_id/rules", tagHandler.ListRules)
 		tags.POST("/:tag_id/rules", tagHandler.CreateRule)
+		tags.PATCH("/:tag_id/rules/:then_tag_id", tagHandler.PatchRule)
 		tags.DELETE("/:tag_id/rules/:then_tag_id", tagHandler.DeleteRule)
 	}
 

backend/internal/handler/tag_handler.go

@@ -354,6 +354,39 @@ func (h *TagHandler) CreateRule(c *gin.Context) {
 	respondJSON(c, http.StatusCreated, toTagRuleJSON(*rule))
 }
 
+// ---------------------------------------------------------------------------
+// PATCH /tags/:tag_id/rules/:then_tag_id
+// ---------------------------------------------------------------------------
+
+func (h *TagHandler) PatchRule(c *gin.Context) {
+	whenTagID, ok := parseTagID(c)
+	if !ok {
+		return
+	}
+
+	thenTagID, err := uuid.Parse(c.Param("then_tag_id"))
+	if err != nil {
+		respondError(c, domain.ErrValidation)
+		return
+	}
+
+	var body struct {
+		IsActive *bool `json:"is_active"`
+	}
+	if err := c.ShouldBindJSON(&body); err != nil || body.IsActive == nil {
+		respondError(c, domain.ErrValidation)
+		return
+	}
+
+	rule, err := h.tagSvc.SetRuleActive(c.Request.Context(), whenTagID, thenTagID, *body.IsActive)
+	if err != nil {
+		respondError(c, err)
+		return
+	}
+
+	respondJSON(c, http.StatusOK, toTagRuleJSON(*rule))
+}
+
 // ---------------------------------------------------------------------------
 // DELETE /tags/:tag_id/rules/:then_tag_id
 // ---------------------------------------------------------------------------