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/models.py

@@ -968,6 +968,8 @@ class ApiToken(Base, TimestampMixin):
     token_hash: Mapped[str] = mapped_column(String(64), unique=True)
     expires_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))  # NULL = never
     last_used_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    # The one notice its user gets before it runs out (`apitokens.warn_expiring`).
+    expiry_warned: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
 
 
 class ApiTokenGrant(Base):