Complete Marketplace Repository

[hayesZach]

Closes CSF-825

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Note

Strengthens update validation and simplifies repository wiring.

• Adds Validate() to MarketplaceUpdates (nil check and name length) with unit tests; private/marketplace.Update now enforces Validate() and propagates its errors
• Updates related tests to expect new error messages; minor cleanup in repository/private/marketplace.go
• Caches key, marketplace, and adminAudit repositories in privateRepository; accessors now return cached instances
• Removes unused service interface files for admin audit, key, marketplace, and reversal

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[linear]

CSF-825 Implement Marketplace Service

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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