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/incidents/[id]/investigation/route.ts

@@ -24,6 +24,13 @@ export async function POST(
   if (incident.status !== 'triaged')
     return NextResponse.json({ error: 'Incident must be triaged first' }, { status: 409 })
 
+  const { count: existingCount } = await supabase
+    .from('investigations')
+    .select('id', { count: 'exact', head: true })
+    .eq('incident_id', id)
+  if ((existingCount ?? 0) > 0)
+    return NextResponse.json({ error: 'Investigation already exists for this incident' }, { status: 409 })
+
   const body = await request.json()
   const method: 'five_why' | 'fishbone' | 'other' = body.method ?? 'five_why'