fix(queue): allow QUEUE_WORKERS env var to override hardcoded worker count

[scnerd]

Summary

The QUEUE_WORKERS env var is supposed to control how many queue workers get created. This is handled by internal/queue/queue.go:getOptimalWorkerCount which checks:

	// Check for environment variable override
	if workerStr := os.Getenv("QUEUE_WORKERS"); workerStr != "" {
		if workers, err := strconv.Atoi(workerStr); err == nil && workers > 0 {
			return workers, workers // Fixed worker count
		}
	}

However, when this value gets used, it's overwritten by a legacy workers argument without checking if the env var was set in internal/queue/queue.go:NewTaskQueue:

	min, max := getOptimalWorkerCount()
	if legacyWorkers > 0 {
		min = legacyWorkers
		max = legacyWorkers
	}

Since NewTaskQueue is called unconditionally with legacyWorkers = 2 (see main.go:130):

	taskQueue := queue.NewTaskQueue(2, unifiedProcessor, jobRepo) // 2 workers

this means that QUEUE_WORKERS is always ignored.

The right solution would seem to be to remove the legacyWorkers argument entirely, since the behavior of selecting worker pool size ranges and auto-scaling is now handled in getOptimalWorkerCount() instead of in main.go. However, the fact that this seemingly-simple refactor wasn't done when getOptimalWorkerCount was first added suggests that it's not as simple as it seems, and as a new contributor, I'll err on the side of caution. As such, this PR only proposes ignoring legacyWorkers when QUEUE_WORKERS is explicitly set. I've left a TODO comment recommending that legacyWorkers be properly deprecated and removed when practical.

This development followed TDD. Tests were verified to fail before changes and work after them.

Fixes #379

Fixes #417

Fixes #428

Disclaimer: Used Claude Code to find bug and implement changes. All code and comments are fully manually reviewed.

[Iconic06]

This would also fix #417 and #428

[scnerd]

Stuck as WIP while I work through manual testing. This doesn't seem to have resolved the issue in my local setup yet.

[scnerd]

Issue was with my testing (was forgetting to build docker image from the correct source), not with the code.

Demo of QUEUE_WORKERS=1: