feat(rt-import): import Red Team engagement results as validated tests

Backend — POST /tests/import-rt (red_lead + admin):
  Accepts engagement JSON with name/date/description/operator and
  a list of techniques each with mitre_id, result, attack_success,
  platform, notes. Creates one Test per technique directly in
  'validated' state (red + blue validation = approved) bypassing
  the normal workflow. Recalculates technique.status_global for
  all affected techniques. Returns created/skipped summary.

Frontend — /tests/import-rt (new dedicated page):
  - Format reference panel (collapsible) with field descriptions
  - Download template JSON button (generates a filled example)
  - Paste JSON textarea + file upload (.json)
  - Live validation + preview table showing what will be imported
  - Import button with spinner
  - Success / warning / error result display
  Accessible to admin and red_lead only.
  Added to sidebar under Tests > Import RT Results.

frontend/src/components/Sidebar.tsx

@@ -22,6 +22,7 @@ import {
   GitCompareArrows,
   ScrollText,
   ClipboardCheck,
+  Upload,
 } from "lucide-react";
 import { useAuth } from "../context/AuthContext";
 import { getTechniques } from "../api/techniques";
@@ -56,6 +57,7 @@ const mainLinks: NavItem[] = [
       { to: "/tests", label: "All Tests", icon: ListChecks },
       { to: "/tests/validated", label: "Validated Tests", icon: CheckCircle },
       { to: "/test-catalog", label: "Test Catalog", icon: BookOpen },
+      { to: "/tests/import-rt", label: "Import RT Results", icon: Upload, roles: ["admin", "red_lead"] },
     ],
   },
   { to: "/campaigns", label: "Campaigns", icon: Zap },