Add more hashtags to tweets

Add all the hashtags from the tags in the post

No file or test is named in the issue. Start by tracing where tweets are assembled from Stack Overflow posts, then identify how post tags are available there. Done means tweets include all tags from the source post as hashtags, with the existing tweet-generation checks or a focused manual verification confirming the result.

索引モデルが issue の本文から書いたものです。