Directory: let an admin give a blog its category

Almost no blog names a category the Directory can use, so a feed can
carry one of its own in config.toml, set by an admin in the feed's
settings and used when the feed names none. The feed's own iTunes
category still wins. The field offers the categories the Directory
already shows, so a blog about games joins Games rather than starting a
second chip. Setting it on a feed from an OPML promotes it to config, as
any other shared setting does.

Closes #10.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/configuration.md

@@ -97,6 +97,7 @@ url                = "https://atp.fm/rss"
 folder             = "Accidental Tech Podcast"   # default: the feed title
 schedule           = "every 6h"                  # overrides [general] for this feed
 media_types        = ["audio"]                   # overrides [general] for this feed
+category           = "Technology"                # the Directory's, if the feed names none
 username           = "ray"                       # HTTP basic auth
 password_env       = "IPX_ATP_PASS"              # preferred over a literal `password`
 ```