An OPML's page lists its shows' items together, not the shows (#283)

GET /api/feeds/{id}/entries for an OPML or a Patreon creator now lists the
items of the shows in it the person subscribes to, as All Subscriptions does
every feed's. The shows are looked up first and queried by list: an OR with a
subquery, or an IN over one, made a single feed's page a sequential scan of
entries (clarkesworld 10 ms to 26 ms or a second, EXPLAIN on production), so a
single feed keeps its `feed_id = $1`.

The page draws the OPML with its own header and the item list, tabs, search
and sort of any feed, with the feed column showing which show each is from.
A show's scan or change refreshes its OPML's list.

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

crates/ipx/CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- `GET /api/feeds/{id}/entries` for an OPML subscription or a Patreon creator lists the items of the shows in it that you subscribe to, together, where it listed nothing (#283).
 - A TypeSafe key is checked with TypeSafe before it is saved, and one it refuses is not saved (#271).
 - A new Signal reads the last week instead of the last 30 days, unless an admin sets otherwise (#276).
 - Editing a Signal's name, words or description asks again only about what it had picked out, and about new items as they come, instead of the whole window again (#276).