POST Initiate Privacy Withdrawal

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OVERVIEW
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Endpoint: POST /api/v1/withdrawal/initiate
Method: POST
Authentication: Not Required
Rate Limit: 30 requests/minute

PURPOSE:
Initiates a privacy withdrawal by validating secret note, generating Merkle proof,
and returning transaction parameters.

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REQUEST BODY
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{
  "secretNote": "cipherx-zk-0.1-eyJzIjoiMHgxYjI5...",
  "recipientAddress": "0xdef456abc123...",
  "poolId": "cipherx-bsc-01",
  "clientId": "web-dapp-v1.0"
}

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RESPONSE
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{
  "success": true,
  "withdrawalId": "wdr_cx_0987654321fedcba",
  "transactionData": {
    "to": "0xE4f77e1E0d6dE17c310FdFA2c088D40222Ab787b",
    "data": "0x...",
    "gasLimit": "700000"
  },
  "withdrawalDetails": {
    "grossAmount": "0.1",
    "platformFee": "0.001",
    "netAmount": "0.099"
  }
}