Keep a feed's items and files to the people who subscribe to it (#129)

Routes that take a feed or an enclosure id did not check who was asking. Anyone signed in could
read any feed's items through GET /api/feeds/{id}/entries, a paid feed's included, with the
addresses of its files, which can carry the subscriber's key: the Directory leaves such feeds
out for that reason, and this route handed them back to whoever guessed the id, a slug of the
title. In production it answered 25 items of a feed the asking account does not subscribe to.
/media/{id} served any downloaded file by its sequential id, POST /api/enclosures/{id}/download
and /api/feeds/{id}/download-latest queued any feed's downloads, and DELETE
/api/enclosures/{id}?force=true deleted any file.

Each now answers 404, "you do not subscribe to that feed", unless the person subscribes to it.
A feed inside an OPML has a subscription row of its own for everyone subscribed to the OPML, so
that holds for those feeds too. Found while adding the Directory's feed page (#128), which has
its own route that answers only for listed feeds and carries no files.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -43,6 +43,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - An iPhone adding the site to its home screen finds the icon at the first address it tries.
 - A feed whose server hangs no longer holds up every scan: a feed gets 30 seconds, and connecting anywhere 10.
 
+### Security
+
+- A feed's items and files reach only the people who subscribe to it. Anyone signed in could read any feed's items, a paid feed's included, with the addresses of its files, which can carry the subscriber's key, and could play, download or delete its files.
+
 ## [0.9.1] - 2026-09-29
 
 ### Added