Purge Fly.io proxy cache after docs release (#154)

## Summary
- The Fly.io nginx proxy caches docs responses for 24h (`proxy_cache_valid 200 1d`)
- After a release, docs.eblu.me kept serving stale content until the cache expired
- This caused v1.5.4 to show v1.5.3 on the CHANGELOG page
- Adds `flyctl` install and `fly ssh console` cache purge steps to the build workflow, running after the ArgoCD deploy completes

## Test plan
- [ ] Next release should show the correct version on docs.eblu.me/CHANGELOG immediately
- [ ] Verify the `fly ssh console` command succeeds in the workflow logs

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/154

.forgejo/workflows/build-blumeops.yaml

@@ -322,6 +322,19 @@ jobs:
 
           echo "Docs app synced successfully!"
 
+      - name: Install flyctl
+        run: |
+          curl -L https://fly.io/install.sh | sh
+          echo "/root/.fly/bin" >> "$GITHUB_PATH"
+
+      - name: Purge Fly.io proxy cache
+        env:
+          FLY_API_TOKEN: ${{ secrets.FLY_DEPLOY_TOKEN }}
+        run: |
+          echo "Purging nginx cache on Fly.io proxy..."
+          fly ssh console -a blumeops-proxy -C "rm -rf /tmp/cache && nginx -s reload"
+          echo "Cache purged"
+
       - name: Summary
         run: |
           VERSION="${{ steps.version.outputs.version }}"