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.

backend/app/main.py

@@ -58,6 +58,7 @@ from app.routes import (  # noqa: E402 — logging is configured first
     setup as setup_routes,
     sync,
     tags,
+    tokens as tokens_routes,
     version,
 )
 from app.scheduler import shutdown_scheduler, start_scheduler  # noqa: E402 — logging is configured first
@@ -337,6 +338,7 @@ app.include_router(plex_routes.router)
 app.include_router(downloads.router)
 app.include_router(downloads.admin_router)
 app.include_router(files_routes.router)
+app.include_router(tokens_routes.router)
 app.include_router(public_routes.router)
 app.include_router(admin.router)
 app.include_router(config_routes.router)