feat: add daily container rescan workflow (#764)

Add a scheduled Trivy vulnerability scan of the published Docker Hub
image (netboxlabs/pktvisor:latest-develop). Renders a vulnerability
table in the step summary and uploads SARIF to GitHub Code Scanning.

PR-time build scanning is deferred due to the expensive C++ build
process. This rescan catches newly disclosed CVEs in the published
image.

Author: marc-barry

.github/workflows/container-rescan.yaml

@@ -0,0 +1,68 @@
+name: Scheduled Container Rescan
+
+on:
+  schedule:
+    - cron: '0 6 * * *'
+  workflow_dispatch:
+
+jobs:
+  rescan:
+    name: Rescan Container Image
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    permissions:
+      actions: read
+      contents: read
+      security-events: write
+    env:
+      TRIVY_DISABLE_VEX_NOTICE: "true"
+      IMAGE_REF: docker.io/netboxlabs/pktvisor:latest-develop
+
+    steps:
+      - name: Pull image
+        run: docker pull "$IMAGE_REF"
+
+      - name: Scan for vulnerabilities
+        uses: aquasecurity/trivy-action@57a97c7e7821a5776cebc9bb87c984fa69cba8f1 # v0.35.0
+        with:
+          image-ref: ${{ env.IMAGE_REF }}
+          format: json
+          output: trivy-output.json
+          vuln-type: os,library
+          scanners: vuln,secret
+          severity: CRITICAL,HIGH,MEDIUM,LOW
+          ignore-unfixed: true
+          exit-code: "0"
+
+      - name: Build rescan summary
+        uses: actions/github-script@ed597411d8f924073f98dfc5c65a23a2325f34cd # v8.0.0
+        with:
+          script: |
+            const fs = require('fs');
+            const data = JSON.parse(fs.readFileSync('trivy-output.json', 'utf8'));
+            const vulns = (data.Results || []).flatMap(r =>
+              (r.Vulnerabilities || []).map(v => ({ ...v, target: r.Target }))
+            );
+            const SEVERITY_LABEL = { CRITICAL: '🔴 **CRITICAL**', HIGH: '🟠 **HIGH**', MEDIUM: '🟡 MEDIUM', LOW: '⚪ LOW' };
+            let summary = `### Rescan Complete\n\n**Image:** \`${process.env.IMAGE_REF}\`\n**Scanned at:** ${new Date().toISOString().replace('T', ' ').split('.')[0]} UTC\n\n`;
+            if (vulns.length === 0) {
+              summary += '_No vulnerabilities found._\n';
+            } else {
+              summary += '| Library | CVE | Severity | Installed | Fixed | Title |\n|---|---|---|---|---|---|\n';
+              for (const v of vulns) {
+                const title = (v.Title || '').replace(/\|/g, '\\|').substring(0, 80);
+                const cve = v.PrimaryURL ? `[${v.VulnerabilityID}](${v.PrimaryURL})` : v.VulnerabilityID;
+                summary += `| ${v.PkgName} | ${cve} | ${SEVERITY_LABEL[v.Severity] || v.Severity} | ${v.InstalledVersion} | ${v.FixedVersion || 'N/A'} | ${title} |\n`;
+              }
+            }
+            fs.appendFileSync(process.env.GITHUB_STEP_SUMMARY, summary);
+
+      - name: Convert scan results to SARIF
+        if: "!cancelled()"
+        run: trivy convert --format sarif --output trivy-results.sarif trivy-output.json
+
+      - name: Upload SARIF to GitHub Code Scanning
+        if: "!cancelled()"
+        uses: github/codeql-action/upload-sarif@0d579ffd059c29b07949a3cce3983f0780820c98 # v4.32.6
+        with:
+          sarif_file: trivy-results.sarif