Complete select-winners API for all supported methods

[3m1n3nc3]

Problem

Winner selection endpoint supports only part of the required selection methods and has ambiguous input behavior.

Current State

• Method handling is random/manual only.
• Manual mode accepts mixed IDs without strict contract validation.
• Merit-based and first-come parity logic is incomplete.

Acceptance Criteria

• Define strict request schema per method (entry IDs vs user IDs).
• Add explicit support for required selection modes and deterministic validations.
• Prevent duplicate or out-of-scope winner assignment in a single transaction.
• Emit post status and notifications consistently after winner selection.

[Mistersmile4585]

@Mistersmile4585 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi maintainers, I’m a fullstack developer with years experience and i will love to work on this issue. Thanks in advance.

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[boys-cyberhub]

@boys-cyberhub has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Please assign this issue to me

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Congratulations, @boys-cyberhub! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

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[drips-wave]

This issue has been completed by @boys-cyberhub as part of the Stellar Wave Program's 4th Wave 🥳

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