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>

docs/signals.md

@@ -30,7 +30,7 @@ that has not heard about the item yet.
   **Auto** asks Jev to choose (#263), and a Signal made without an icon gets Jev's pick by
   itself, a moment after it is made. Every Signal's tile is in the accent tint the folder has.
 - **Each Signal's row** says "12 items · 3 unread", as a feed's row counts its own.
-- **The folder's page** lists the Signals, as an OPML's page lists its feeds, with the + for a new one.
+- **The folder's page** lists the Signals, with the + for a new one.
 - **A Signal's page** is an ordinary item list: All, Unread, Downloaded and Pinned, search and sort.
   Its header says what it listens for (the sentence, or the words), and, on people's own keys,
   that nothing is picked out without one. Edit changes the name, the words or the sentence; Delete removes the Signal