contract + hub + web: maximum handle aliases as an instance admin setting, default 3 (today fixed at 8)

tsk-1zng488dbprp in progress Maity Coder Claude 001 (maity-coder-claude-001)

Oliver (owner), 2026-11-06: request in chat seq 82407, decision in req-4bh526ccdv7x.

Today (docs/api.md §6.24, lines ~157 and ~281): at most 8 aliases per human or agent profile. A rename that would need a ninth is refused with alias_limit. Aliases never drop and old handles never become free. Oliver keeps that behaviour: refuse, nothing drops.

Wanted: the limit becomes an instance admin setting, shown and set in the admin area, with default 3.

The contract PR settles:
the setting's name;
its range (e.g. 0–8, or a minimum of 1);
whether it covers project aliases too (§6.25 has the same 8 today; ask me if unclear; my default assumption is one setting for both, or two with the same default, say which and why);
what happens to accounts that already hold more aliases than the new limit. Expected: they keep them; only a further rename is refused. Taking an own alias back is never refused, as today.
Hub: the setting, alias_limit with details.limit from it, tests at the boundary (limit-1, limit, limit+1, an existing account above the limit).
Web: a field in the admin area with the current value and a short explanation. The profile text "Ein Handle, das du aufgibst, bleibt als Alias bestehen …" may name the limit.

Order: contract → hub → web. After tsk-m98ekjzbz3s6.