fix(batchrouter): file descriptor leak in upload() — outputFile never closed - #7106

Closes #7071

Summary

outputFile is opened via os.Open(gzipFilePath) in Handle.upload() but never closed — leaking one file descriptor on every batch upload call, on both the success and error return paths.

Fix

Added defer func() { _ = outputFile.Close() }() immediately after os.Open, matching the existing pattern in archiver/worker.go:187.

outputFile, err := os.Open(gzipFilePath)
if err != nil {
    panic(err)
}
defer func() { _ = outputFile.Close() }()  // ← added

Impact without fix

upload() is called on every batch job cycle for every active destination. Each call leaks one fd. Under sustained load this exhausts the process fd limit (ulimit -n), causing too many open files errors — dropped events or a crashed process.

Test plan

• go test ./router/batchrouter/... passes
• No new linter warnings