Docs and site: who pays for Signals, and Signals in the API reference (#271, #269)

docs/signals.md says what the admin's setting does and that a key is
checked before it is saved; the install guide lists TYPESAFE_KEY; the API
reference has Signals' routes and types and the new fields of Me and
Settings; the home page says an admin may provide the key.

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

site/index.html

@@ -98,7 +98,7 @@
     <div class="feature">
       <div class="say">
         <h3>Tune out the noise</h3>
-        <p>Your feeds bring in more than anyone has time for. A Signal listens across all of them for one subject: give it a name, the words to listen for, and a sentence saying what you mean if the words are not enough. What fits lands in a list of its own, starting with the last month, and it keeps listening. To use Signals you will need an API key from <a href="https://typesafe.ai">typesafe.ai</a>.</p>
+        <p>Your feeds bring in more than anyone has time for. A Signal listens across all of them for one subject: give it a name, the words to listen for, and a sentence saying what you mean if the words are not enough. What fits lands in a list of its own, starting with the last month, and it keeps listening. To use Signals you will need an API key from <a href="https://typesafe.ai">typesafe.ai</a>, unless your server's admin provides one.</p>
       </div>
       <figure class="frame"><img class="cut" src="/shots/signals.webp" width="874" height="398" loading="lazy"
         alt="A Signal called Space, picked out of the noise of eight feeds: episodes and posts about galaxies, Artemis II, Mars and a rocket launch from Hackaday, Universe Today, NASA's Houston We Have a Podcast and Ars Technica."></figure>