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]/addenda/route.ts

@@ -13,6 +13,10 @@ export async function GET(
   const { data: { user }, error: authError } = await supabase.auth.getUser()
   if (authError || !user) return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
 
+  const { data: profile } = await supabase.from('users').select('role').eq('id', user.id).single()
+  if (!profile || !['hse', 'admin', 'supervisor'].includes(profile.role))
+    return NextResponse.json({ error: 'Forbidden' }, { status: 403 })
+
   const { data, error } = await supabase
     .from('incident_addenda')
     .select('id, body, created_at, author:users!author (name)')