feat(admin): export audit trail as CSV and strictly validate enterprise list queries

[Ayush7614]

What

Admin > Audit was a read-only screen: the only way to review the trail was to look at it. This adds the missing take-out path plus query-parity fixes in the same admin surface:

1. Audit CSV export — GET /api/admin/audit-events?format=csv answers the same filtered query (eventType, actorUserId, initiatorKind, targetType, targetId, from, to) as RFC 4180 CSV, with an Export CSV button on the Audit page that downloads the page's current filter.
2. Strict enterprise list queries — GET /api/admin/enterprise/models/usage?days= and GET /api/admin/enterprise/actions?limit= used Number(raw) || default, so ?days=abc, ?days=12abc, ?limit=3.9 silently returned default views while the audit/channel/people lists answer 400. Both now parse strictly (400 naming the parameter, clamp 1..365 / 1..500), matching parsePageLimit.

Why

A compliance pull, an incident timeline, or a count of what policy refused had nowhere to go — admins could only screenshot the table. The export unblocks evidence collection, and the strict parsing closes a silent-coercion inconsistency where a typo returned a plausible-looking wrong page.

What's in it

• server/src/audit.ts: AUDIT_EXPORT_COLUMNS, auditEventsToCsv (RFC 4180 escaping, decision/bot columns, redacted payload as one JSON column), readAuditExportRows (walks matching pages via the same reader the JSON list uses; ignores list paging; caps at 5,000 rows and reports truncation).
• server/src/app.ts: ?format=csv branch with text/csv + Content-Disposition: attachment, X-Audit-Export-Truncated header when capped, 400 on unknown format. JSON path untouched.
• server/src/admin/routes.ts: exported pure parseEnterpriseDays / parseEnterpriseActionsLimit wired into both GETs (400s at the edge; store clamps stay as defence in depth).
• app/src/lib/audit/queries.ts + audit.tsx: auditExportUrl(search) (filter travels, paging dropped, stray format replaced) and an Export button with loading/error states.
• docs/architecture.md: short "Taking the trail out" subsection.
• Tests: server/tests/audit-export.test.ts (serializer, collector incl. truncation, route: headers/body/400s/JSON-unchanged), server/tests/enterprise-list-query-validation.test.ts (defaults, clamps, 400 cases), app/tests/audit-export-url.test.ts (filter/paging/format handling).

Verification (all run locally, all green)

• New tests: 20 pass across the 3 new files.
• Surrounding suites: 106 pass across audit (cursor, limit, query, retention, initiator), enterprise-controls, and app audit tests.
• bunx biome lint --error-on-warnings: clean on all 8 touched source/test files.
• bun run --filter server typecheck and bun run --filter app typecheck: both exit 0.
• Note: a fresh bun install was needed in this checkout before tests could resolve workspace deps (pre-existing, unrelated to this change). No dependencies added or changed by this PR.

Original work for this repo: no open issue or PR covers audit export (checked open issues/PRs before starting); branch is fresh off current upstream/main (17831b2) with nothing else on it.