feat(files): API tokens in Settings, the Hub folder in Files, a notice before a token runs out

Settings gains an API tokens tab: a row per token (name and prefix; each
folder with its rights; last use and expiry, "expires soon" marked),
revoked from its menu behind a confirm. A new token is the Hub exchange in
one click, or folders and rights picked per folder (no Write where an
upload cannot land); expiry 30/90 days, 1 year or never (warned). The
token shows once in the same dialog, with Copy and a ready curl line.

The Hub folder closes the Sources in the Files rail, takes uploads and
moved-in files. A token about to run out warns its user once, a week
before (the Files GC's sixth pass); the notice opens the tokens tab.

backend/app/routes/tokens.py

@@ -64,6 +64,7 @@ def _serialize(db: Session, token: ApiToken) -> dict:
         "created_at": iso(token.created_at),
         "expires_at": iso(token.expires_at),
         "expired": token.expires_at is not None and token.expires_at <= now,
+        "expiring": apitokens.expiring(token, now),
         "last_used_at": iso(token.last_used_at),
         "grants": [
             {