fix(code-quality): clarify sanitizer errors and vault thresholds

[Dopezapha]

Closes #309
Closes #312

PR Description

This PR fixes two small backend code-quality issues: #309 and #312.

For #309, vault deposit thresholds are now named constants instead of inline numbers, making the maximum safe deposit and large-deposit alert cutoff easier to understand and maintain.

For #312, input sanitizer validation failures now return clearer messages that describe the expected format and include examples where useful. Stellar public key and contract ID validation now uses the Stellar SDK StrKey validators, so G-address and C-address inputs are checksum-aware instead of only shape-checked.

These changes improve reviewer and API-consumer clarity without changing the intended vault deposit flow.

Changed

• #606 Extract magic numbers in vaults.service.ts to named constants #309: Extracted MAX_SAFE_DEPOSIT and LARGE_DEPOSIT_THRESHOLD in vaults.service.ts.
• #609 Improve error messages in InputSanitizerService #312: Improved sanitizer error messages for Stellar keys, contract IDs, UUIDs, emails, amounts, and strings.
• #609 Improve error messages in InputSanitizerService #312: Used Stellar SDK StrKey validators for Stellar public keys and contract IDs.
• #609 Improve error messages in InputSanitizerService #312: Added focused sanitizer unit coverage.

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