fix: interaction-screening and bulk-prediction

[vtnphan]

Pull Request

Summary

SBP-660 interaction-screening and bulk-prediction (WISPS) launches never deducted credits — launch_credit_cost() only priced single-prediction/de-novo-design and returned None for the rest, so the deduction at launch was a silent no-op.

Changes

• launch_credit_cost() (credits.py) now prices bulk-prediction (multiplier × entry count) and interaction-screening (multiplier × query count × target count).
• Added _count_wisps_samplesheet_entries() (workflows.py) to derive that quantity server-side from the samplesheet already staged for the run (via its group column), instead of trusting the client-supplied fastaContent — keeps the charge non-spoofable.
• Updated a stale docstring on SbpCreditField (admin.py).
• No migrations or backwards-incompatible changes.

How to Test

1. uv run pytest tests/test_routes_workflows.py — covers bulk/interaction-screening deduction and 402 rejection on insufficient credits.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added or updated documentation where necessary
• I have run linting and unit tests locally
• The code follows the project's style guidelines

[amandazhuyilan]

Nice fix - promise me you will do a test once this goes into dev?

[vtnphan]

Nice fix - promise me you will do a test once this goes into dev?

Yes, sure thing!