chore: tidy up uploading migration code (#12577)

- Validate and sanitize topics.
- Cap topics at 25 (limit used elsewhere, now unified constant).
- Add more details and rephrase common user-facing error messages.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12577

routers/api/v1/repo/topic.go

@@ -104,7 +104,7 @@ func UpdateTopics(ctx *context.APIContext) {
 	topicNames := form.Topics
 	validTopics, invalidTopics := repo_model.SanitizeAndValidateTopics(topicNames)
 
-	if len(validTopics) > 25 {
+	if len(validTopics) > repo_model.MaxTopicsPerRepo {
 		ctx.JSON(http.StatusUnprocessableEntity, map[string]any{
 			"invalidTopics": nil,
 			"message":       "Exceeding maximum number of topics per repo",

routers/web/repo/topic.go

@@ -29,7 +29,7 @@ func TopicsPost(ctx *context.Context) {
 
 	validTopics, invalidTopics := repo_model.SanitizeAndValidateTopics(topics)
 
-	if len(validTopics) > 25 {
+	if len(validTopics) > repo_model.MaxTopicsPerRepo {
 		ctx.JSON(http.StatusUnprocessableEntity, map[string]any{
 			"invalidTopics": nil,
 			"message":       ctx.Tr("repo.topic.count_prompt"),