feat: add CVE tooltip on hover in Reporting Page

- Add GET /api/cves/:cveId/tooltip backend endpoint with description truncation
- Create CveTooltip portal component with caching, severity badges, and viewport-aware positioning
- Integrate tooltip into ReportingPage with 300ms hover delay on CVE badge spans

.kiro/specs/cve-tooltip-hover/.config.kiro

@@ -0,0 +1 @@
+{"specId": "b8855eb4-3949-426e-86ac-36fe069a6bb1", "workflowType": "requirements-first", "specType": "feature"}