fix: add server-side validation for critical API endpoints

[Oby38]

Closes #43

This PR adds explicit request validation for the serverless API endpoints in api/calculate-score.js, api/evaluate-and-mint.js, api/get-available-credit.js, and api/get-user-data.js.

Accepted request shapes:

• POST /api/calculate-score with { address: string, totalDeposited?: number }
• POST /api/evaluate-and-mint with { userAddress: string, totalVolume?: number, deposits?: [{ amount: number }] }
• POST /api/get-available-credit with { userAddress: string }
• GET /api/get-user-data?address=<string>

Rejected invalid cases:

• missing required address or userAddress
• empty string values for required addresses
• non-numeric or negative values for optional volume fields
• invalid deposits array items for evaluate-and-mint

A shared helper was introduced at api/validation.js with Zod schemas and consistent 400 responses for bad requests.

Validation performed:

• npm run build succeeded after installing dependencies
• direct handler invocation tests showed malformed input returns 400 for each endpoint

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