Trim the state database; Popular lists feeds the way Directory does

Drops the created columns on users, subscriptions and sessions, which were
written by every insert and read by nothing, and migrate()'s add list, whose
columns all predate 0.3.0. Removes Db::subscribed_feed_ids (no callers),
Db::subscriber_count (one caller wanting > 0) and Managed.orphaned (never
read). The old-database test now builds the tables with foreign keys on.

Popular now lists the feeds inside an OPML or a Patreon creator, never the
collection, as Directory does.

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

CHANGELOG.md

@@ -12,8 +12,10 @@ The long form, with what was wrong before and how it was found, is in
 
 ### Changed
 
-- Directory lists the feeds inside an OPML one by one, and no longer the OPML itself, so you can
-  subscribe to just the shows you want. Popular still counts an OPML as one feed.
+- Directory and Popular list the feeds inside an OPML one by one, and no longer the OPML itself,
+  so you can subscribe to just the shows you want.
+- The database no longer records when accounts, subscriptions and sign-ins were created. Nothing
+  ever read it, and an existing database drops the columns on its next start.
 
 ## [0.5.1] - 2026-09-12