chore(open-notebook): position as admin-side; document single-tenant

open-notebook has no user accounts upstream (PasswordAuthMiddleware = one
shared password; SurrealDB has no user/account/owner table — verified in the
running image). One shared password = one shared workspace; the guard chain
gates access but cannot partition data, and Authentik identity is never
consumed. Multi-user is approved-but-unbuilt upstream (#712, post-v2); OIDC
declined (#607). So move the homepage card from the user grid to the admin
tier and add the single-tenant boundary to the vault page. Fix the card's
stale 'frank-zealot' -> frank-archon. Update the seam's incidental category
pin to the new intentional value. Password rotated on the host (not repo).

tests/open_notebook.bats

@@ -290,7 +290,9 @@ vhost = sys.argv[2]
 cards = [s for s in data if s["name"] == "open-notebook"]
 assert len(cards) == 1, f"expected exactly one open-notebook card, got {len(cards)}"
 card = cards[0]
-assert card["category"] == "user" and card["section"] == "AI", card
+# category moved to "admin": single shared password, no per-user accounts, so it
+# is positioned as an admin-side tool, not a user-facing app (see the vault page).
+assert card["category"] == "admin" and card["section"] == "AI", card
 assert card["probe"] == "127.0.0.1:8502", card["probe"]
 assert card["method"] == "http"
 # Deployed and guarded: the card is live and its link is the vhost, so the