Add stress test harness for concurrency bugs

[knifecake]

Summary

• Adds a standalone stress test script (tests/stress_test.py) that spins up the full supervisor with configurable workers/threads/queues, enqueues thousands of tasks, and verifies correctness invariants (no lost jobs, no duplicates, no stuck executions).
• Adds a stress_counter_task to the dummy test app with configurable workload profiles (--workload none|light|medium|heavy) mixing IO-bound (sleep) and CPU-bound (SHA-256 iterations) work.
• Quiets the django.tasks logger in test settings to reduce noise during stress runs.

This is the kind of test that surfaces race conditions like #25 (duplicate ClaimedExecution) and #26 (process is None on execution).

Usage

# Default: 1000 tasks, 3 workers, 4 threads, 3 queues
uv run python tests/stress_test.py

# Full stress: 10k tasks with simulated work
uv run python tests/stress_test.py --tasks 10000 --workers 3 --threads 4 --workload medium

# Quick smoke test
uv run python tests/stress_test.py --tasks 100 --timeout 60 --workload light

Test plan

• Verified with 10 tasks (all workload levels)
• Run with 10k tasks to reproduce IntegrityError Stacktrace encountered on busy systems #25 / 'NoneType' object has no attribute 'name' #26 at scale

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