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.

topics.go

@@ -31,7 +31,7 @@ type Topic struct {
 	CategoryID int    `json:"category_id"`
 	CreatedAt  string `json:"created_at"`
 	Closed     bool   `json:"closed"`
-	Archived  bool   `json:"archived"`
+	Archived   bool   `json:"archived"`
 	PostStream struct {
 		Posts []Post `json:"posts"`
 	} `json:"post_stream"`