Cut what the audit found: dead columns, one-time upgrades, three deps

Works through TODO.md from the 2026-09-12 over-engineering audit. Drops the
entries.read/flagged/position columns (migrate() removes them from older
databases), migrate_opml_children, the legacy interval_mins key, the
contrib/ systemd units, test-only Db wrappers, a duplicate token generator,
redundant logbuf visitors, unused page state and CSS, and the infer, dirs
and tokio-stream dependencies. The icon is served once as /icon.png instead
of inlined four times, taking about 94 KB off the two pages.

The adoption's subscription half was not dead: it gives a fresh install's
first admin the config's feeds. It stays as adopt_catalogue, now tested.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TAC7sLVqfKmY6rsTLXzNgk

docs/configuration.md

@@ -43,8 +43,6 @@ media_types       = ["audio", "video"]
   can be fetched by hand; blog feeds put each article's header image in an `<enclosure>`, and
   without this the disk fills with artwork. Empty takes everything.
 
-`interval_mins` from older configs is still read, and `schedule` supersedes it.
-
 ## `[torrent]`
 
 ```toml