#7542 Cap the default backend worker count

Summary

• Cap the automatically selected Redis-backed backend worker count at 8.
• Preserve the Granian-specific GRANIAN_WORKERS override.
• Clarify worker configuration in deployment docs and add unit coverage for CPU-count boundaries.

Closes #7541

Testing

At commit 2dc6659:

• uv run pytest tests/units/utils/test_processes.py -q (36 passed)
• Applicable pre-commit hooks passed.

The documentation wording was subsequently clarified in commit adf3a98; please rerun pre-commit on the current head.