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/backend.md

@@ -10,6 +10,8 @@ The build mirrors the URL space — hashed immutable assets under `/_assets/`, `
 
 Generated HTML pages (content pages, category/404 placeholders, `/_a`) go through `_html_response`: zstd-compressed per request at level 9 when the client sends `accept-encoding: zstd` (no gzip fallback; static assets are pre-compressed by the `Frontend`), with `vary: accept-encoding` set and the ETag kept identical across encodings so `if-none-match` revalidation still works. In production the rendered bodies are cached in an LRU keyed by everything the output depends on — page kind, path, the site origin (social meta), encoding, and `data.version`, which bumps on every content/settings change and so transparently invalidates the whole cache. The cache is bypassed in dev, where theme/design CSS is re-read from disk per request. Content pages carry an ETag built from the node's modified timestamp and `data.version`; `/_a` instead gets a blake3 hash of the rendered body (it has no Node), with matching `if-none-match` revalidations answered by a 304.
 
+Uploaded files, seed assets and fetched external-site favicons live in the `FileStore`: content-addressed files on disk under `<hostname>/files/` (`PAGERITE_FILES`), fully cached in RAM at startup — both the raw body and a zstd-compressed copy (kept only when smaller). `GET /_f/{name}` serves from the RAM cache with immutable caching, answering the zstd variant when the client accepts it; the name is the ETag. Legacy databases that still carry blobs in a `files` kanta field are migrated to disk by `pagerite/migrations.py::migrate_v1` (kanta's `migrate_vN` mechanism, wired via `Kanta(..., migrations="pagerite.migrations")`), which pops the field from the raw state before struct decoding.
+
 ## `data.py`
 
 msgspec Structs for the kanta database. See `docs/content-model.md` for the full data model.
@@ -24,7 +26,7 @@ markdown-it-py renderer (html passthrough + attrs, footnote, deflist, tasklists,
 
 The shared page layout as an html5tagger `Template` with placeholders (`Title`, `Brand`, `Banner`, `Nav`, `Sidebar`, `Main`), nav rendering straight from the `Data.menu` tree (siblings sorted by `Node.order`; nav links to content-less labels point at their first child via `first_leaf`, the first published descendant with content), and page/404 rendering.
 
-Content pages get SEO/social meta (description, canonical link, Open Graph + twitter card) from heuristics over the rendered article: the description is the first paragraph's text, the share image prefers a `{.hero}`-classed image, then the first raster `<img>`, then the first SVG; the first `<video>` yields `og:video`; URLs are made absolute with the site origin (`Data.site_url` — learned from admin browsers reporting their `location.origin` via `POST /_api/site-url`, correct even behind reverse proxies; until learned, the request's own base URL is the fallback); `article:published/modified_time` come from `Node.created`/`modified`. If the markdown contains its own h1, the page title is NOT rendered as an additional h1 (it still supplies `<title>` and nav labels).
+Content pages get SEO/social meta (description, canonical link, Open Graph + twitter card) from heuristics over the rendered article: the description is the first paragraph's text, the share image prefers a `{.hero}`-classed image, then the first raster `<img>`, then the first SVG; the first `<video>` yields `og:video`; URLs are made absolute with the site origin (`SITE_URL` — `https://<hostname>` from the CLI hostname argument; on localhost the request's own base URL is the fallback); `article:published/modified_time` come from `Node.created`/`modified`. If the markdown contains its own h1, the page title is NOT rendered as an additional h1 (it still supplies `<title>` and nav labels).
 
 The navbar holds top-level items only; the current section's subitems go to a left `#sidebar` as a nested list (the section's direct children plain, deeper levels indented with article-list-style markers), rendered only from the second level down — main-level pages list their children as cards after the content instead. Below that, the sidebar renders when the section offers at least two published items, or exactly one while viewing anything other than that only page — the section index, a 404, a grandchild (so those pages can reach the child), and also on that only page itself when it has published children of its own; no aside element at all on the front page, main-level pages, leaf pages and the sole childless page of a one-page section. Also, category labels are nodes without content — None *or* empty markdown — and their nav links point at their first child page. Dynamic regions have stable ids (`#page-banner`, `#nav`, `#sidebar`, `#main`) for fetch-navigation swaps (`#sidebar` may be absent on either side of a swap).