feat(compliance): add time-based trend charts to Compliance page

Add 6 Recharts charts in a collapsible Historical Trends panel on the
Compliance page, covering all Tier-1 recommendations from the reporting
design doc.

Backend — 5 new API endpoints:
  - GET /api/compliance/trends        — active totals + per-team counts per upload
  - GET /api/compliance/mttr          — mean days to resolution per team
  - GET /api/compliance/top-recurring — most persistent active findings by seen_count
  - GET /api/compliance/category-trend — category breakdown per upload (future use)
  - GET /api/archer-tickets/status-trend — ticket pipeline by creation date + status

Frontend — new ComplianceChartsPanel component:
  - Active Findings Over Time (multi-line: total + per-team dashed)
  - Change per Report Cycle (stacked bar: new/recurring + resolved)
  - Team Compliance Health (multi-line per team)
  - Mean Time to Resolution (horizontal bar per team)
  - Most Persistent Findings (horizontal bar top-10 by seen_count)
  - Archer Exception Pipeline (stacked bar by date + status)

All charts degrade gracefully to a no-data placeholder until uploads
accumulate. Panel is collapsible to stay out of the way when not needed.
Adds recharts dependency to frontend.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/pages/CompliancePage.js

@@ -3,6 +3,7 @@ import { Upload, MessageSquare, RefreshCw, AlertCircle, Loader } from 'lucide-re
 import { useAuth } from '../../contexts/AuthContext';
 import ComplianceUploadModal from './ComplianceUploadModal';
 import ComplianceDetailPanel from './ComplianceDetailPanel';
+import ComplianceChartsPanel from './ComplianceChartsPanel';
 
 const API_BASE = process.env.REACT_APP_API_BASE || 'http://localhost:3001/api';
 const TEAL = '#14B8A6';
@@ -323,6 +324,9 @@ export default function CompliancePage({ onNavigate }) {
                 </div>
             ) : null}
 
+            {/* ── Historical trend charts ──────────────────────────────── */}
+            <ComplianceChartsPanel />
+
             {/* ── Device table ─────────────────────────────────────────── */}
             <div style={{
                 background: 'linear-gradient(135deg,rgba(30,41,59,0.95) 0%,rgba(15,23,42,0.98) 100%)',