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/guides/install.html

@@ -50,6 +50,7 @@ docker build -f Dockerfile.web -t ipx-web .</code></pre>
       <tbody>
         <tr><td><code>IPX_DATABASE_URL</code></td><td>A <code>postgres://user:password@host:5432/database</code> address, to keep everything in Postgres instead of SQLite</td></tr>
         <tr><td><code>PUID</code>, <code>PGID</code></td><td>The user and group that own the files ipx writes</td></tr>
+        <tr><td><code>TYPESAFE_KEY</code></td><td>The server's own <a href="https://typesafe.ai">typesafe.ai</a> API key: it picks a podcast's Directory category and a Signal's icon, and, when an admin turns it on, pays for everyone's Signals</td></tr>
         <tr><td><code>IPX_LOG_FORMAT</code></td><td><code>json</code> logs one JSON object a line, for Loki and the like</td></tr>
         <tr><td><code>OTEL_EXPORTER_OTLP_ENDPOINT</code></td><td>Sends traces of requests, checks and downloads to an OpenTelemetry collector; set <code>WEB_TRACING=on</code> on ipx-web too</td></tr>
         <tr><td><code>IPX_UPSTREAM</code></td><td>On ipx-web: where ipx is, <code>ipx:8099</code> by default</td></tr>