Feature/oauth2 login

[EDOHWARES]

Implement OAuth2 Social Login and Stellar Payment Channel Monitoring

This PR addresses and implements the acceptance criteria for two major backend features:

1. #939: Implement OAuth2 social login with Google and GitHub
2. #937: Implement Stellar payment channel monitoring

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🚀 Key Implementations

1. OAuth2 Social Login (Google & GitHub)

• Added dedicated routes (GET /auth/google, GET /auth/github along with their callback handlers) using NestJS passport strategies (passport-google-oauth20 and passport-github2).
• Linked multiple OAuth providers to a single account using a new entity table, user_oauth_links, avoiding signup friction while ensuring consistency by grouping OAuth links under identical email addresses.
• Ensured uniform JWT access/refresh token pair issuance regardless of authentication method.

2. Stellar Payment Channel Monitoring

• Built StellarClientService to establish a live streaming connection to Stellar Horizon using server.payments().forAccount(vaultFundingAccountId).stream().
• Emits a custom PaymentReceivedEvent domain event within seconds of ledger close.
• Designed automatic connection retry logic using exponential backoff to handle network interruptions or stream drops.
• Integrated stream status monitoring directly into the /health endpoint under the stellar_payment_stream key.
• Gated the DepositConfirmed execution pathway to trigger asynchronously only when the payment stream yields a matching ledger payment.

3. Core Bugs & Test Suite Refactoring

• Fixed Unawaited Promise Bugs: Resolved four security-critical spots where isCurrentUserAdmin was evaluated as a promise in boolean contexts without being awaited (making checks evaluate to true and allowing unauthorized actions).
• Asynchronous Flow Integration in Tests: Refactored all 97 unit and integration tests in vaults.service.spec.ts and vaults.integration.spec.ts to accommodate the change from synchronous/atomic deposits and withdrawals to the new asynchronous CQRS/event-driven confirmation architecture.

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🛠️ Tech Stack & Dependencies Added

• passport-google-oauth20, passport-github2, @nestjs/passport
• Stellar SDK Horizon client stream listeners

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🧪 Verification & Test Results

All Vault-related test suites were run and passed successfully:

PASS src/vaults/vaults.service.spec.ts
PASS src/vaults/vaults.integration.spec.ts

Test Suites: 2 passed, 2 total
Tests:       97 passed, 97 total
Snapshots:   0 total

Closes #467
Closes #465
Closes #464
Closes #466

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[EDOHWARES]

@vic-Gray , pls review