[server] - Security: Staking Endpoints Accept `user_id` from Request Body

[Hydrax117]

Description

StakeBody.user_id and ClaimBody.user_id in staking_handler.rs come from the JSON request body. Any authenticated user can stake or claim rewards on behalf of any other user by supplying a different UUID. There is no verification that the calling user owns the user_id in the payload.

// staking_handler.rs — current (INSECURE)
pub struct StakeBody {
    pub user_id: String,  // ← client-controlled
    pub stellar_address: String,
    pub amount: i64,
}

Acceptance Criteria

• user_id is extracted from JWT claims (via ClaimsExt or web::ReqData<Uuid>), never from the request body
• stellar_address is validated against the authenticated user's registered Stellar account in the database
• A test verifies that supplying a different user's ID in the body returns 403 Forbidden
• unstake/{user_id} path parameter is also validated against JWT identity

Technical Notes

Follow the pattern in reputation_handler::get_my_reputation which already uses web::ReqData<Uuid> correctly.

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[Hovibby]

@Hovibby has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I can deliver an excellent work on this issue sir

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