feat(files): API tokens — a machine reaches the library folder by folder

A user can make a token for a machine of theirs (`/api/tokens`, signed-in only): `sfl_` + 32
random bytes + a checksum, only its SHA-256 kept, shown once. Sent as a Bearer header it is
honoured on six `/api/files` routes only — folders, list, file, hash, upload, delete — and there
only in the folders its grants name, with the rights they give (list / read / write / delete).
One check, `FilesCaller.may`, in every route; a folder or file out of reach answers like a
missing one; the files shared with the user are never in reach. A token beats a cookie on the
same request.

The Hub preset makes the user's one Hub folder (a new folder kind) and grants it every right.
Refused tokens are slowed down per client, a runaway machine per token; last use is stamped at
most once a minute; making and revoking a token is audited, and a token's uploads and deletes
name it in their audit rows.

CLAUDE.md

@@ -62,7 +62,7 @@ how it gets built. Concretely:
 ## Architecture shape (don't fight it)
 
 `routes/*` (HTTP only: validation, auth-dependency, serialization) → domain packages `sync/`, `plex/`,
-`downloads/`, `youtube/`, `issues/` (shared by routes + scheduler + worker) → flat core (`auth`, `quota`,
+`downloads/`, `youtube/`, `issues/` (shared by routes + scheduler + worker) → flat core (`auth`, `apitokens`, `quota`,
 `sysconfig`, `state`, `audit`, `notifications`, `userscope`, `ratelimit`, `bodylimit`, `progress`, `collation`,
 `security`, `logs`, `logsink`, `diagnostics`, `debugsession`) → `models/db/config`. **Dependency direction is one-way: core never imports routes.**
 Frontend: `main.tsx` provider tree → `App.tsx` shell (a shared `/watch` link boots from its own