Marketplace Repository

[zedimytch]

This PR adds the MarketplaceRepository implementation and corresponding tests.

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Note

Marketplace repository implementation and validations

• Add internal/repository/private/marketplace.go with Create/Read/Update/Delete, including transactional delete that cascades keys; comprehensive tests in marketplace_test.go and SQLite in-memory test harness in internal/testutil.
• Strengthen models.Marketplace validation (length/regex) and wire GORM relations in models.Key (json:"id", foreign key to Marketplace).

DTO layer and interface shifts

• Introduce internal/domain/dto package: MarketplaceUpdateOptions (+ToFields, Validate), Key{Update,List}Options, ReversalListOptions, AdminAuditListOptions, and Cursor (+ protobuf under dto/cursorpb).
• Update repository/service interfaces to use dto types instead of ad-hoc structs and move RawKey to dto.

Environment/type consolidation and secrets

• Move Environment to models/constants and update config, secret, and related interfaces to reference it; adjust secret key prefixes accordingly.

Misc

• Add github.com/google/go-cmp for testing; enable migrations ordering in private repo; new helper DB seed/migration in tests.

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