Fix: Enable RQ_QUEUES configuration via environment variables (#8455)

[seeker-jie]

Reason for change

Problem: RQ_QUEUES was hardcoded to an empty dictionary {} in label_studio/core/settings/label_studio.py, preventing Redis Queue
configuration via environment variables and breaking background job processing.

Solution: Changed RQ_QUEUES = {} to RQ_QUEUES = json.loads(get_env("RQ_QUEUES", "{}")) to enable environment variable
configuration while maintaining backward compatibility.

Rollout strategy

This change uses an existing environment variable pattern (get_env) and maintains backward compatibility with empty dict default. No
feature flags required - the change is safe to deploy immediately.

Testing

Manual verification:

• Verified RQ_QUEUES defaults to {} when RQ_QUEUES environment variable is not set
• Verified RQ_QUEUES correctly parses JSON configuration from RQ_QUEUES environment variable
• Tested that existing deployments without the env var continue working unchanged

Unit tests: Not required for this change as it's a simple Django settings configuration modification. Existing RQ functionality tests
will verify the configuration works correctly.

Risks

Low risk:

• Backward compatible change - existing deployments unaffected
• Uses established get_env pattern consistent with other settings in the same file
• JSON parsing could theoretically fail with malformed input, but this follows the same pattern as other env var configurations

Reviewer notes

• Single line change in settings file
• Follows existing code patterns in the same file (see other get_env usage)
• Addresses issue raised in [bug] RQ_QUEUES configuration ignores environment variables and defaults to empty dict #8455
• No additional tests needed - this is a configuration-level change

General notes

This fix enables proper Redis Queue configuration for production deployments where Redis might be running on different hosts/ports or
require authentication, while maintaining the current behavior for existing installations.

Fixes #8455

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