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/package.json

@@ -12,6 +12,7 @@
     "react-dom": "^19.2.4",
     "react-markdown": "^10.1.0",
     "react-scripts": "5.0.1",
+    "recharts": "^3.8.1",
     "web-vitals": "^2.1.4",
     "xlsx": "^0.18.5"
   },