style: gofmt struct-tag alignment across v0 files

v0 landed with a few misaligned struct tags and fields; gofmt -l
flagged categories.go, topics.go, discourse_test.go and the smoke
fake. Alignment only, no behavior change, so the tree passes the
gofmt gate cleanly.

smoke/fakediscourse/main.go

@@ -69,7 +69,7 @@ func main() {
 			name, _ := req["name"].(string)
 			cat := map[string]any{
 				"id": s.nextCat, "name": name,
-				"slug": strings.ToLower(strings.ReplaceAll(name, " ", "-")),
+				"slug":  strings.ToLower(strings.ReplaceAll(name, " ", "-")),
 				"color": orDefault(req["color"], "3AB54A"), "text_color": orDefault(req["text_color"], "FFFFFF"),
 				"topic_count": 0,
 			}