feat: Upload companion agent to SeaweedFS CDN + update download links

CI pipeline now uploads binaries to cdn.corrosionmgmt.com:
- /companion/latest/ (always current, overwritten each release)
- /companion/v1.x.x/ (versioned archive)

Frontend download links updated from Gitea releases to CDN.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/build-companion.yml

@@ -85,6 +85,39 @@ jobs:
             --data-binary @companion-agent/bin/checksums.txt \
             "${API_URL}/repos/${REPO}/releases/${RELEASE_ID}/assets?name=checksums.txt"
 
+      - name: Upload to CDN (latest)
+        run: |
+          CDN_URL="https://cdn.corrosionmgmt.com"
+
+          # Upload Linux binary to /companion/latest/
+          curl -s -X POST \
+            -F "file=@companion-agent/bin/corrosion-companion-linux-amd64" \
+            "${CDN_URL}/companion/latest/corrosion-companion-linux-amd64"
+
+          # Upload Windows binary to /companion/latest/
+          curl -s -X POST \
+            -F "file=@companion-agent/bin/corrosion-companion-windows-amd64.exe" \
+            "${CDN_URL}/companion/latest/corrosion-companion-windows-amd64.exe"
+
+          # Upload checksums
+          curl -s -X POST \
+            -F "file=@companion-agent/bin/checksums.txt" \
+            "${CDN_URL}/companion/latest/checksums.txt"
+
+          # Also upload versioned copies
+          VERSION=${{ steps.version.outputs.VERSION }}
+          curl -s -X POST \
+            -F "file=@companion-agent/bin/corrosion-companion-linux-amd64" \
+            "${CDN_URL}/companion/${VERSION}/corrosion-companion-linux-amd64"
+          curl -s -X POST \
+            -F "file=@companion-agent/bin/corrosion-companion-windows-amd64.exe" \
+            "${CDN_URL}/companion/${VERSION}/corrosion-companion-windows-amd64.exe"
+          curl -s -X POST \
+            -F "file=@companion-agent/bin/checksums.txt" \
+            "${CDN_URL}/companion/${VERSION}/checksums.txt"
+
+          echo "CDN upload complete: ${CDN_URL}/companion/latest/"
+
       - name: Build Summary
         run: |
           echo "## Companion Agent Build Complete" >> $GITHUB_STEP_SUMMARY