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/route.ts

@@ -15,6 +15,17 @@ export async function POST(request: Request) {
   if (authError || !data?.user) return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
   const user = data.user
 
+  const since = new Date(Date.now() - 60_000).toISOString()
+  const { count: recentIncidents } = await supabase
+    .from('audit_log')
+    .select('id', { count: 'exact', head: true })
+    .eq('changed_by', user.id)
+    .eq('table_name', 'incidents')
+    .eq('action', 'INSERT')
+    .gte('changed_at', since)
+  if ((recentIncidents ?? 0) > 0)
+    return NextResponse.json({ error: 'Rate limited — please wait 60 seconds before submitting another incident' }, { status: 429 })
+
   let body: Record<string, unknown>
   let files: File[] = []