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/audit.py

@@ -69,6 +69,10 @@ class AuditAction:
     FILE_FOLDER_RENAME = "file_folder_rename"
     FILE_FOLDER_RETENTION = "file_folder_retention"
     FILE_FOLDER_DELETE = "file_folder_delete"
+    # A machine's token to the library (`routes.tokens`): made (its folders and rights) or revoked.
+    # What a token then did is the file rows above, their summary naming it.
+    API_TOKEN_CREATE = "api_token_create"
+    API_TOKEN_REVOKE = "api_token_revoke"
 
 
 def record(