feat: Add multi-process worker support with Redis-based task storage

[Ahmad-cercli]

Summary

Implements multi-process worker support by storing task execution details in Redis instead of in-memory dictionaries.

Problem

Current implementation stores task payloads (func, args, kwargs) in process-local memory (worker_pool._tasks), which breaks when running multiple worker processes:

• Process A submits task → stores in Process A's memory
• Process B's worker picks up task via ZPOPMIN → can't find execution details in Process B's memory
• Task orphaned with status "queued" but never processed

This makes Kew incompatible with common production patterns like Uvicorn/Gunicorn with --workers N > 1.

Solution

Store task payloads in Redis using cloudpickle serialization, enabling any worker process to execute any task.

Key Changes

1. Task Payload Storage (submit_task)

• Serialize func/args/kwargs with cloudpickle.dumps()
• Store in Redis as task_payload:{task_id} with base64 encoding (for decode_responses=True compatibility)
• Expires after 24 hours (configurable via TASK_EXPIRY_SECONDS)

2. Task Execution (_process_queue)

• Load payload from Redis instead of worker_pool._tasks
• Deserialize with cloudpickle.loads()
• Clean up payload after task completes

3. Circuit Breaker Configuration

• Added max_circuit_breaker_failures parameter to QueueConfig
• Allows effectively disabling circuit breaker for multi-process setups where process-local circuit breakers don't coordinate

4. Cleanup Improvements

• Automatic deletion of task_payload:* keys after task completion
• Added TASK_PAYLOAD_PREFIX constant for consistency

Backward Compatibility

• ✅ Single-process deployments work unchanged
• ✅ Existing QueueConfig parameters compatible (new param has default)
• ✅ No breaking API changes
• ✅ Existing tests pass

Testing

Tested in production with:

• 4 Uvicorn workers processing 100+ concurrent tasks
• No duplicate task executions observed
• Tasks correctly distributed across all worker processes
• OCR and AI screening workloads running for 1 week stable

Dependencies

• Added cloudpickle>=3.0.0 for robust function/closure serialization

Checklist

• ✅ Tests pass
• ✅ Code formatted with Black
• ✅ Imports sorted with isort
• ✅ No linting errors
• ✅ CHANGELOG.md updated

Related Issues

Implements roadmap item: "Distributed workers with coordination (locks) for multi-process scaling"

[justrach]

Hey! The CI is failing due to deprecated GitHub Actions versions in the workflow file. I've just pushed a fix to main that updates them to v4/v5. Could you rebase your branch on main to pick up the fix? Thanks!

[justrach]

LGTM