Per-hostname data directory <hostname>/{content.kantadb, analytics.json, files} from CLI arg; drop learned site_url.

The first positional CLI argument (default localhost) names the site's
public hostname and its data directory under the cwd, replacing the
CWD-relative pagerite.* files. The public origin (https://<hostname>)
is now authoritative configuration instead of a value learned from
admin browsers: POST /_api/site-url, Data.site_url and the pagerite.js
reporter are removed, and page rendering, sitemap, robots.txt and
analytics own_origin all use SITE_URL consistently (localhost falls
back to the request's base URL).

docs/frontend-runtime.md

@@ -22,6 +22,10 @@ The `::view-transition*` block at the end of `pagerite.css` (from termotohtori.f
 
 Vite builds ES-module `.js` outputs; in dev the backend links them as `<script type="module">` (module scripts defer by default), in production it inlines them at the end of the body.
 
-## Database file
+## Data directory
 
-The database file is `pagerite.kantadb` in the cwd (`PAGERITE_DB` overrides); gitignored. Do not delete it without asking.
+All site data lives under `<hostname>/` in the cwd — `content.kantadb`,
+`analytics.json` and `files/` — where `<hostname>` is the CLI's first
+positional argument (default `localhost`, exported as `PAGERITE_HOSTNAME`;
+`PAGERITE_DB`/`PAGERITE_ANALYTICS`/`PAGERITE_FILES` override individual
+paths). gitignored. Do not delete it without asking.