Adopt nuqs for audit-log-screen filter/pagination state

[stephane-segning]

Summary

admin/app/audit-log/audit-log-screen.tsx hand-rolls its filter and
pagination state with plain useState (model, actorId, offset), even
though nuqs (2.9.5) is already a dependency and already used for exactly
this purpose — URL-persisted, bookmarkable filter state — in
messages-screen.tsx, jobs-screen.tsx, and webhooks-screen.tsx. This
screen appears to have simply been missed when those three adopted it.

Source of truth: docs/design/frontend-package-audit.md ("nuqs for
audit-log-screen.tsx — adopt" section), from the frontend package audit
requested by the maintainer.

Scope

In admin/app/audit-log/audit-log-screen.tsx, replace:

const [model, setModel] = useState("");
const [actorId, setActorId] = useState("");
const [offset, setOffset] = useState(0);

with useQueryStates (nuqs), matching the pattern already established in
admin/app/jobs/jobs-screen.tsx (parseAsString/parseAsStringEnum for the
filters; the offset can use parseAsInteger). Nothing else about the screen
changes — this is a state-management substitution, not a UX change, though
the resulting URL-shareable filter state is a real, incidental improvement
(an operator can now bookmark or share a specific audit-log filter view, the
same way they already can for messages/jobs/webhooks).

Small and mechanical: roughly 15–20 lines of URL-sync boilerplate removed,
replaced by the existing nuqs pattern already proven three times over in
this codebase.

Properties that must survive this change

• Pagination (offset) resets to 0 when a filter changes, matching the
  current behavior — check the existing jobs-screen.tsx pattern for how
  it handles this (its filter change handlers explicitly reset paging
  state) and replicate it here rather than assuming nuqs does it for free.
• The screen's existing hasFilters/"clear filters" affordance (if present)
  keeps working against the new nuqs-backed state.

Verification

• pnpm --filter admin typecheck stays green.
• Manually exercise: setting a filter updates the URL query string; a
  bookmarked/shared URL with ?model=...&actorId=... reproduces the same
  filtered view on load; changing a filter resets offset to 0.

AI Usage Declaration

This issue was filed by an AI agent (Claude) performing a maintainer-
requested audit of packages/* for hand-rolled machinery a well-maintained
library could replace. The scope was derived by reading
audit-log-screen.tsx and comparing it directly against the already-adopted
nuqs pattern in jobs-screen.tsx/messages-screen.tsx/webhooks-screen.tsx,
not inferred. A human should confirm the scope above and verify the manual
checks before merging.

• Human has reviewed the scope above
• Human confirms the URL-state behavior was verified live before merge