[BACKEND] Paginated API route for gallery

[yrska]

🎯 Acceptance Criteria

• Returns 200 with { images: [...], hasMore: true } when more pages exist
• Returns 200 with { images: [...], hasMore: false } when on the last page
• Returns 200 with { images: [], hasMore: false } when no gallery images exist
• Returns 400 for a request missing page or limit params
• Returns 400 for non-numeric page or limit value
• Uses a mocked Payload client so tests do not require a running CMS instance
• Test file lives in app/api/gallery/route.test.ts

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📊 Definition of Done (DoD)

• Acceptance criteria met
• Agreed tests written
• Agreed documentation in GitHub wiki
• PR reviewed

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🔗 Dependencies

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📚 Additional Context