Do not store `bad_files` & `hashfield` in `sites.json`

Because these make `sites.json` too bloat, and slow down whole client.
There should be stored in `content.db` instead.

Start by locating the code that writes and reads sites.json, bad_files, hashfield, and content.db; the issue names no specific files or tests. Trace how these values are used across the client before deciding the migration scope. Done means bad_files and hashfield are no longer stored in sites.json, are stored in content.db, and the client retains its behavior without the reported bloat.

索引モデルが issue の本文から書いたものです。