Encapsulate all migrations in kanta migrate_vN, drop persisted Data.version

migrate_v1 now also rebuilds the legacy flat pages store as the menu
tree (moved from the app lifespan, raw-dict level); migrate_v2 now also
backfills missing AVIF/WebP/JPEG derivatives on disk (moved from the
lifespan) and drops the obsolete version field. The version render
counter was cache-invalidation state, not database state: replaced by an
in-memory render generation that clears the page-body LRU and feeds page
ETags. The legacy Page struct and Data.pages/version fields are removed;
old databases lose the stale keys on re-serialization.

docs/content-model.md

@@ -8,13 +8,13 @@ The site structure is stored in the kanta database managed by `pagerite/data.py`
 
 `Node.content` is the Markdown page, or None for a pure category label whose URL renders a 404 listing its children as cards (while nav links to it point at its first child); every label's title and slug are editable. A page with published children — a category page — lists them as cards after its markdown content; the sidebar sub-navigation renders only from the second level down, never on main-level pages.
 
-Siblings order by the fractional `Node.order` key: a moved item gets a fresh key relative to its new siblings, all others keep theirs. `resolve`/`find_slot` walk the tree by path; moves are slot detach/attach carrying the whole subtree. Legacy flat `Data.pages` (pre-tree databases) migrates into `menu` on startup. The app owns the `Data` object; reads are plain attribute access, writes in `kanta.transaction(...)`.
+Siblings order by the fractional `Node.order` key: a moved item gets a fresh key relative to its new siblings, all others keep theirs. `resolve`/`find_slot` walk the tree by path; moves are slot detach/attach carrying the whole subtree. Legacy flat `pages` (pre-tree databases) migrates into `menu` via `migrate_v1`. The app owns the `Data` object; reads are plain attribute access, writes in `kanta.transaction(...)`.
 
-`Data.version` is bumped on every write and embedded in page ETags so nav-affecting changes invalidate caches.
+Every content/settings write calls `_invalidate_pages()` in app.py, which clears the rendered-body LRU and bumps an in-memory render generation embedded in page ETags, so nav-affecting changes invalidate caches. (This used to be a persisted `Data.version` counter — cache invalidation is not database state, so the field was dropped; old databases lose the key on re-serialization.)
 
 ## Files
 
-Files are content-addressed (blake3[:12] + extension) and stored **on disk** under `<hostname>/files/` (path from `PAGERITE_FILES`), served at `/_f/{name}` with immutable caching. Uploaded raster images (except GIF) and SVGs (rasterized) get a set of derivatives: the untouched original under `<hash>.orig<ext>` (internal only — it may carry EXIF data and is never served; SVG originals stay servable as `<hash>.svg`), a mediapreview-recompressed AVIF (`<hash>.avif`, thumbnailed to `IMAGE_MAXSIZE` at `IMAGE_QUALITY`), and WebP/JPEG fallbacks re-encoded from the AVIF at lower quality (`IMAGE_WEBP_QUALITY`/`IMAGE_JPG_QUALITY`, chosen for similar-or-smaller file size). Pages link the bare `/_f/<hash>` and the server negotiates by Accept header: a format is served only when listed explicitly (`image/avif` → AVIF, `image/webp` → WebP, anything else including `image/*` and `*/*` → JPEG); an explicit extension in the URL pins the format. Responses carry `vary: accept`. Favicons uploaded in settings go through the same pipeline at `FAVICON_MAXSIZE` (192px). Existing databases are updated by `migrate_v2` (link rewrite) plus a startup backfill that creates missing derivatives. Deleting any name of a hash removes the whole group. The `FileStore` in app.py caches every file in RAM, both uncompressed and zstd-compressed (the compressed copy only when smaller), so `/_f` answers both encodings without disk reads. Pages reference files by absolute `/_f/` URLs so hierarchy moves never break them. Pre-refactor databases kept the blobs in a `Data.files` kanta field; the kanta migration `pagerite/migrations.py::migrate_v1` writes them to disk on open and drops the field (removed from `Data`). Fetched favicons of external analytics sites live in the same store (see `docs/analytics.md`).
+Files are content-addressed (blake3[:12] + extension) and stored **on disk** under `<hostname>/files/` (path from `PAGERITE_FILES`), served at `/_f/{name}` with immutable caching. Uploaded raster images (except GIF) and SVGs (rasterized) get a set of derivatives: the untouched original under `<hash>.orig<ext>` (internal only — it may carry EXIF data and is never served; SVG originals stay servable as `<hash>.svg`), a mediapreview-recompressed AVIF (`<hash>.avif`, thumbnailed to `IMAGE_MAXSIZE` at `IMAGE_QUALITY`), and WebP/JPEG fallbacks re-encoded from the AVIF at lower quality (`IMAGE_WEBP_QUALITY`/`IMAGE_JPG_QUALITY`, chosen for similar-or-smaller file size). Pages link the bare `/_f/<hash>` and the server negotiates by Accept header: a format is served only when listed explicitly (`image/avif` → AVIF, `image/webp` → WebP, anything else including `image/*` and `*/*` → JPEG); an explicit extension in the URL pins the format. Responses carry `vary: accept`. Favicons uploaded in settings go through the same pipeline at `FAVICON_MAXSIZE` (192px). Existing databases are updated by `migrate_v2` (link rewrite plus on-disk derivative backfill). Deleting any name of a hash removes the whole group. The `FileStore` in app.py caches every file in RAM, both uncompressed and zstd-compressed (the compressed copy only when smaller), so `/_f` answers both encodings without disk reads. Pages reference files by absolute `/_f/` URLs so hierarchy moves never break them. Pre-refactor databases kept the blobs in a `Data.files` kanta field; the kanta migration `pagerite/migrations.py::migrate_v1` writes them to disk on open and drops the field (removed from `Data`). Fetched favicons of external analytics sites live in the same store (see `docs/analytics.md`).
 
 ## Banners