feat: create StellarService to centralize Horizon client — eliminate duplication across modules

[Josue19-08]

🔖 Title

Create `StellarService` — centralize Horizon client instantiation currently duplicated across modules

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📄 Description

`src/blockchain/stellar/` exists in the architecture but contains only a `.gitkeep`. The Stellar Horizon client is instantiated independently in at least two places, with diverging retry strategies and no shared error normalization:

• `src/modules/transactions/transactions.service.ts` — creates its own `StellarSdk.Horizon.Server` instance inline
• `src/jobs/transaction-status-checker/transaction-status-checker.processor.ts` — creates a second independent instance with its own inline retry loop using different timing constants

This duplication means:

• Horizon URL and network passphrase are read from env vars in two different places
• Retry/back-off logic differs between the two — a bug fix in one doesn't fix the other
• No single place to add observability hooks, circuit breaking, or connection pooling
• Unit tests must mock `StellarSdk` globally instead of injecting a service

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✅ Tasks to complete

• Create `src/blockchain/stellar/stellar.service.ts` — wraps `Horizon.Server` with: configurable base URL from `ConfigService`, unified exponential back-off retry (max 3 attempts), normalized error types (`StellarNetworkError`, `TransactionNotFoundError`), and timeout per request
• Create `src/blockchain/stellar/stellar.module.ts` — exports `StellarService` as a global module
• Refactor `src/modules/transactions/transactions.service.ts` — inject `StellarService`; remove inline `Horizon.Server` instantiation
• Refactor `src/jobs/transaction-status-checker/transaction-status-checker.processor.ts` — inject `StellarService`; remove duplicated inline retry loop
• Update `src/jobs/transaction-status-checker/transaction-status-checker.module.ts` — import `StellarModule`
• Update `src/modules/transactions/transactions.module.ts` — import `StellarModule`
• Add unit tests for `StellarService` in `test/unit/blockchain/stellar/stellar.service.spec.ts` covering: successful fetch, retry on transient error, failure after max retries, transaction not found

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📚 Documentation/context for AI

(This link never should removed)
https://github.com/TrustUp-app/TrustUp-API/tree/main/docs

Relevant files:

• `src/blockchain/stellar/` (currently empty — create here)
• `src/modules/transactions/transactions.service.ts`
• `src/jobs/transaction-status-checker/transaction-status-checker.processor.ts`
• `src/jobs/transaction-status-checker/transaction-status-checker.module.ts`
• `src/modules/transactions/transactions.module.ts`

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🗒️ Additional notes

• Follow the same NestJS module pattern used by `SupabaseModule` (`src/database/supabase/`) — make it a `@Global()` module so all consumers just import it once
• Normalized error types should extend `Error` and carry the original Horizon error for logging
• The retry strategy should use exponential back-off: 500ms → 1000ms → 2000ms with jitter
• Network passphrase (`STELLAR_NETWORK_PASSPHRASE`) must be added to `src/config/env.ts` validation if not already present

[Chigybillionz]

Greetings, TrustUp team.

I'd like to work on this issue.

I'll create a reusable "StellarService" and "StellarModule" to centralize Horizon client creation, unify retry and timeout behavior, normalize Stellar errors, and refactor the existing transactions service and status checker to use dependency injection. I'll also add the required unit tests, update the module wiring and environment validation, and ensure the implementation aligns with the documented architecture.

GitHub: https://github.com/chiggybillionz