harden auth email validation

[saurabhhhcodes]

Summary

• Normalize and validate auth email inputs before they reach MongoDB queries
• Reject malformed email payloads in signup, verify, resend-OTP, and signin flows
• Trim the service-layer lookup too, so the query path stays safe even if future callers pass padded input

Validation

• git diff --check
• node --check server/src/routes/auth.js
• node --check server/src/services/userService.js
• npm run test:auth:unit in server

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