feat: add input validation and sanitization to all API endpoints (#338)

[Julianemeka]

Added comprehensive input validation and sanitization to all API endpoints using Zod.

Changes:

• Enforced Zod schema validation on all request bodies, path parameters, and query parameters.
• Applied .trim() to all string fields to prevent whitespace issues and basic injection.
• Enforced strict numeric bounds (e.g., kwh > 0, limit within 1-100).
• Validated UUID formats for all ID parameters.
• Standardized error responses to return 400 with descriptive validation errors.
• Applied validation before any business logic in every route.

Closes #338

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