Signals: feeds made from your subscriptions, picked by Jev (#254)

A Signal is a name and keywords. A task beside DeadAir's asks TypeSafe's Jev, on the owner's own
key, about each item of their subscribed feeds from the last 30 days and every new one: one call
an item, one noul question for each of their Signals still to hear about it, 0.5 or more a fit.
Every verdict is kept, fitting or not, in signal_verdicts, so nothing is asked twice, and new
keywords drop a Signal's verdicts to be judged afresh. A key TypeSafe refuses (401/402/403) is
passed over until its owner saves another, so a bad key is not sent the whole queue.

A Signal's items come through entries_where with an EXISTS on its hits, so filters, search, sort
and read state are an ordinary list's, and reading an item in a Signal reads it in its feed.

The page shows Signals under the places, with New signal, and Settings takes the TypeSafe key,
which is never sent back. The browser suite stubs Jev in the fixture server (IPX_TYPESAFE_URL).

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

web/CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Signals, under the places in the sidebar: New signal makes one from a name and keywords, and its page lists the items from your feeds that are about them, read and unread as in their own feeds. Settings takes your TypeSafe key, which Signals need (#254).
 - A list opens at once when you have seen it before or the page fetched it ahead: All Subscriptions and the feeds with something unread are fetched when the page loads, and a feed or filter as the pointer or a finger reaches it, then refreshed behind once shown. A list that has to be waited for shows a spinner instead of nothing (#247). Currently Listening and the Directory's front page are fetched ahead the same way and open at once (#248).
 
 ### Changed