feat: add asset inventory page for discovered hosts and services

[aaniya22]

Closes #38

What's changed

• Added AssetInventory.jsx page under frontend/src/pages/ showing all discovered hosts and services
• Added /inventory route in routes.ts and App.tsx
• Added Inventory link to desktop sidebar and mobile nav
• Added GET /api/v1/assets endpoint in backend/secuscan/routes.py
• Wired up getAssets() in api.ts using the existing request() helper

Features

• Table with host/IP, open ports, scanner, finding count, highest severity, first/last seen timestamps, and tags
• Live search filter across host, IP, port, tag, scanner
• Severity and scanner dropdowns
• Sort by host, findings, severity, or last seen
• Click any row to expand a detail panel
• Empty state handled
• Stats strip showing total hosts, unique ports, findings, and last scan time

Testing

• Manually verified with backend running
• Empty state shows correctly when no scans have been run