fix: credit cost

[vtnphan]

Pull Request

Summary

SBP-660 interaction-screening and bulk-prediction (WISPS) launches never deducted credits, and the admin "SBP Credit" field had no way to show a cost for them even once fixed, since their charged quantity isn't a stored field.

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, instead of trusting the client-supplied fastaContent. Keeps the charge non-spoofable.
• Added WorkflowRun.credit_cost, persisted at launch with the exact amount charged. SbpCreditField (admin.py) now reads it directly instead of recomputing from final_design_count, so the admin display works for all four workflow categories and stays accurate even if pricing changes later.
• New migration 0a8ae645d1b3 adds workflow_runs.credit_cost. Regenerated docs/schema_diagram(.svg).

How to Test

1. uv run pytest — covers bulk/interaction-screening deduction, 402 rejection on insufficient credits, and the persisted credit_cost value.
2. uv run python generate_migrations.py --check to verify the migration applies cleanly.

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]

Promise me you do a test on dev after merge?

[vtnphan]

Promise me you do a test on dev after merge?

Yep, will test again