fix: guard against merging a tag into itself to prevent destructive deletion

apps/web/modules/projects/settings/lib/tag.test.ts

@@ -195,6 +195,20 @@ describe("tag lib", () => {
         });
       }
     });
+    test("returns error when merging a tag into itself", async () => {
+      const result = await mergeTags(baseTag.id, baseTag.id);
+      expect(result.ok).toBe(false);
+
+      if (!result.ok) {
+        expect(result.error).toStrictEqual({
+          code: "merge_same_tag",
+          message: "Cannot merge a tag into itself",
+        });
+      }
+
+      expect(prisma.tag.findUnique).not.toHaveBeenCalled();
+      expect(prisma.$transaction).not.toHaveBeenCalled();
+    });
     test("throws on prisma error", async () => {
       vi.mocked(prisma.tag.findUnique).mockRejectedValueOnce(new Error("fail"));
       const result = await mergeTags(baseTag.id, newTag.id);

apps/web/modules/projects/settings/lib/tag.ts

@@ -72,6 +72,13 @@ export const mergeTags = async (
 ): Promise<Result<TTag, { code: TagError; message: string; meta?: Record<string, string> }>> => {
   validateInputs([originalTagId, ZId], [newTagId, ZId]);
 
+  if (originalTagId === newTagId) {
+    return err({
+      code: TagError.MERGE_SAME_TAG,
+      message: "Cannot merge a tag into itself",
+    });
+  }
+
   try {
     let originalTag: TTag | null;
 

apps/web/modules/projects/settings/types/tag.ts

@@ -1,5 +1,6 @@
 export enum TagError {
   TAG_NOT_FOUND = "tag_not_found",
   TAG_NAME_ALREADY_EXISTS = "tag_name_already_exists",
+  MERGE_SAME_TAG = "merge_same_tag",
   UNEXPECTED_ERROR = "unexpected_error",
 }