Improve the user/wallet creation process

[dappsar]

User Duplication

• Issue: The code checks if a user already exists but doesn't handle cases where the process is interrupted between the verification and the wallet creation, potentially leading to duplicate users.
• Improvement: Implement database transactions to ensure atomicity of the process, or use mechanisms like optimistic locking to prevent duplication.

Lack of Robust Phone Number Verification

• Issue: Phone numbers are validated only by their length, which may lead to inconsistencies if malformed numbers are entered.
• Improvement: Implement more robust phone number validation, such as enforcing the E.164 format.

Lack of Robust Error Handling (related with: #26)

• Issue: In several parts of the code, such as wallet creation or token issuance, error handling is not properly implemented, which can result in unexpected failures.
• Improvement: Enhance error handling and fallbacks to ensure that failures are logged and managed properly without compromising the user flow.