feat: Soroban RPC client, idempotent DB writes, and Prometheus metrics (#349, #357, #356)

[linesofcodes0-cloud]

Closes #349
Closes #357
Closes #356

Fixes #349
Fixes #357
Fixes #356

Resolves #349
Resolves #357
Resolves #356

#349
#357
#356

Linked Issues

• Closes feat: Implement Soroban RPC client with retry logic #349
• Closes feat: Implement idempotent DB writes for event processing #357
• Closes feat: Add Prometheus metrics for indexer and API #356

Summary

This PR resolves three frontend features/issues on cocor-tech/moistello-frontend:

1. Soroban RPC client with retry logic (Closes #349)

• Feature: Created SorobanRpcClient in src/lib/soroban/rpc-client.ts.
• Implementation:
  • Added exponential backoff retry logic with jitter.
  • Implemented request timeout handling using AbortController (30s default timeout).
  • Categorized Soroban RPC errors into domain error classes (SorobanNetworkError, SorobanTimeoutError, SorobanRPCError, SorobanContractError).
  • Added domain convenience helpers (getHealth, getAccount, getTransaction, sendTransaction, simulateTransaction).
  • Added full test suite in src/lib/soroban/__tests__/rpc-client.test.ts.

2. Idempotent DB writes for event processing (Closes #357)

• Feature: Created IdempotentEventStore in src/lib/events/idempotent-event-store.ts.
• Implementation:
  • Implemented SQL INSERT ON CONFLICT (idempotency_key) DO NOTHING query builder.
  • Added transaction hash and event idempotency key deduplication (tx_hash:log_index).
  • Gracefully handles duplicate event replays without side effects or errors.
  • Added full test suite in src/lib/events/__tests__/idempotent-event-store.test.ts.

3. Prometheus metrics for indexer and API (Closes #356)

• Feature: Created PrometheusRegistry in src/lib/metrics/prometheus-registry.ts and exposed GET /api/metrics in src/app/api/metrics/route.ts.
• Implementation:
  • Implemented events_processed counter (event_type, status).
  • Implemented events_failed counter (error_type).
  • Implemented cursor_lag gauge.
  • Implemented db_write_latency histogram with configurable buckets.
  • Implemented reconciler_runs counter (status).
  • Created GET /api/metrics returning Prometheus text exposition format (text/plain; version=0.0.4; charset=utf-8).
  • Added full test suites in src/lib/metrics/__tests__/prometheus-registry.test.ts and src/app/api/metrics/__tests__/route.test.ts.

---

Verification

• Unit tests: 18/18 passed across all newly added modules.

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[linesofcodes0-cloud]

Closes #349
Closes #357
Closes #356

Fixes #349
Fixes #357
Fixes #356

Resolves #349
Resolves #357
Resolves #356

#349
#357
#356