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

src/download.rs

@@ -223,7 +223,7 @@ enum Sniffed {
 /// 2008 and so always answered 'data'.
 async fn sniff(path: &Path) -> Result<Sniffed> {
     let head = read_head(path, 512).await?;
-    if infer::is(&head, "torrent") || head.starts_with(b"d8:announce") || head.starts_with(b"d7:") {
+    if head.starts_with(b"d8:announce") || head.starts_with(b"d7:") {
         return Ok(Sniffed::Torrent);
     }
     let text = String::from_utf8_lossy(&head);