fix(validation): introduce shared stellar address validator for auth, accounts, escrow, and subscriptions (#16)

[ghzhost]

Overview

Fixes #16 by introducing a centralized, checksum-validating validate_stellar_address helper and enforcing it across registration (/api/auth/register), account lookup (/api/accounts/:address), escrow creation, and subscription creation.

Context & Problem

Previously:

• src/routes/auth.rs::register accepted optional stellar_address and stored it directly without validation.
• src/routes/accounts.rs had a rudimentary private length+prefix check without checksum verification.
• src/services/escrow.rs and src/services/subscription.rs checked only non-empty strings for accounts.

Solution

1. Shared Validator Module (src/validation.rs):
   • Uses stellar_strkey::ed25519::PublicKey::from_string for standard Ed25519 public keys (G...).
   • Uses stellar_strkey::ed25519::Med25519PublicKey::from_string for SEP-23 multiplexed keys (M...).
   • Rejects empty, malformed, or checksum-invalid addresses with structured AppError::Validation.
2. Consistent Integration:
   • auth::register: Validates req.stellar_address if provided.
   • accounts: Uses validation::validate_stellar_address.
   • escrow::create: Validates depositor_account, beneficiary_account, and optional arbiter_account.
   • subscription::create: Validates payer_account and recipient_account.
3. Unit Tests:
   • Added unit test suite in src/validation.rs covering valid G keys, invalid checksums, invalid prefixes, and empty inputs.

Closes #16