Add a vote casting endpoint that submits the cast_vote contract call and records the vote in the database

[Chucks1093]

Summary

There is no server endpoint for holders to cast governance votes through the client. A POST /keys/:keyId/proposals/:proposalId/vote endpoint that validates the holder's eligibility, submits the contract call, and persists the vote should be added.

Scope

• Add POST /keys/:keyId/proposals/:proposalId/vote requiring a valid JWT
• Accept optionIndex in the request body and validate it is within the proposal's option range
• Check that the caller has not already voted on the proposal and return 409 if they have
• Submit the cast_vote contract call and write the vote record to proposal_votes
• Return 404 if the proposal does not exist or is already closed

Acceptance Criteria

• Valid vote submitted to contract and persisted
• Duplicate vote returns 409
• optionIndex out of range returns 422
• Vote on a closed proposal returns 404
• Non-holder JWT with zero balance returns 403

ETA: 12 hours

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[crookdev11]

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[Mabel-003]

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[bashir347]

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[Mona-i]

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