Vendor credit alert workflow

[dotnetspark]

Summary

Add a non-blocking vendor credit alert workflow for invoice create and approve actions.

Scope

• Pre-create a CreditCheckRecord with a deterministic credit_check_id
• Run the credit evaluation after the response using FastAPI BackgroundTasks
• Compute outstanding AP from invoices in PENDING, MATCHED, and APPROVED
• Persist a single active vendor-level CreditAlert when the credit limit is exceeded
• Expose GET /credit-checks/{id} from the invoices routing surface
• Extend GET /vendors/{id}/exposure to include the active alert when present
• Preserve idempotent replay so the same logical request returns the same credit_check_id without duplicate background work

Constraints

• The triggering create and approve responses remain otherwise unchanged except for top-level credit_check_id
• Credit-risk signaling is advisory only and must never block the triggering action
• Background task exceptions are caught and logged silently

References

• specs/018-vendor-credit-alert/spec.md
• specs/018-vendor-credit-alert/plan.md
• specs/018-vendor-credit-alert/tasks.md