Add minimumCost field to illConfig

[kurtnordstrom]

No description provided.

[Copilot]

Copilot review overview

🟢 Approval recommended

The field is consistently implemented across the contract, database, API, import workflow, and regression tests.

Review effort: Balanced
Findings: None

What changed in this PR

Adds nullable, non-negative minimumCost support across Directory configuration persistence, APIs, and imports.

Changes:

• Adds schema and database support for minimumCost.
• Preserves PATCH semantics and validates negative values.
• Adds API, persistence, and import coverage.

File	Description
directory/​api.yaml	Defines minimumCost in API and import schemas.
directory/​api/​catalog_config.go	Maps create and PATCH values to persistence.
directory/​api/​entries.go	Includes the field in entry responses.
directory/​api/​loan_config_test.go	Tests PATCH and null semantics.
directory/​import/​model/​config.go	Adds the import model field.
directory/​import/​db/​entry.go	Persists imported values.
directory/​import/​db/​repo_test.go	Verifies imported database persistence.
directory/​import/​service/​importer_test.go	Verifies import decoding.
directory/​query.sql	Extends the ILL configuration upsert.
directory/​migrations/​013_minimum_cost.up.sql	Adds the constrained database column.
directory/​migrations/​013_minimum_cost.down.sql	Removes the column on rollback.
directory/​test/​entries_test.go	Tests round-trip and partial PATCH behavior.
directory/​test/​loan_config_test.go	Tests validation and persistence through HTTP.

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