fix: P1 API security hardening — rate limits, auth guards, duplicate prevention

- verify/route.ts: setDate → setUTCDate to avoid timezone off-by-one on recheck date
- triage-suggest, rca-draft, quality-check: 60s per-user rate limit via audit_log
- quality-check: add write_audit_log (was missing, CLAUDE.md violation)
- investigation POST: 409 if investigation already exists for incident
- incidents POST: 60s per-user rate limit via audit_log
- addenda GET: restrict to hse/admin/supervisor roles
- dashboard/stats GET: restrict to hse/admin/management roles

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CPf5Rc8QPx6V8KLEEgfKEQ

app/api/capa/[id]/verify/route.ts

@@ -31,7 +31,7 @@ export async function POST(
 
   const verifiedAt = new Date()
   const recheckDate = new Date(verifiedAt)
-  recheckDate.setDate(recheckDate.getDate() + 30)
+  recheckDate.setUTCDate(recheckDate.getUTCDate() + 30)
 
   const update: Record<string, unknown> = {
     status: body.verdict,