Define model structs for records, communities, licenses, and stats

[ran-codes]

Description

Define all Go struct types that represent Zenodo API response objects.

Requirements

• Supports FR-2.x (records), FR-4.x (communities), FR-5.x (access links), FR-6.x (licenses)

Acceptance Criteria

• internal/model/record.go — Record, Deposition, Metadata, Creator, RelatedIdentifier structs with JSON tags
• internal/model/community.go — Community struct
• internal/model/license.go — License struct
• internal/model/stats.go — Stats struct (downloads, views, unique_downloads, unique_views, version_downloads, version_views)
• internal/model/error.go — already exists from Phase 1, verify it covers all error shapes
• All structs have proper json tags matching Zenodo API field names
• Pagination wrapper: PaginatedResponse[T] or equivalent for list endpoints

Dependencies

• Blocked by: Phase 1 completion (needs API client)