feat: add dependency probes to /api/indexer/health

[adesuwa-tech]

Summary

Add Postgres and Soroban RPC dependency probes to the GET /api/indexer/health endpoint alongside the existing cursor lag check.

Changes

• src/routes/indexer/health.ts: Added injectable dependency probes for database (Postgres SELECT 1) and Soroban RPC (getLatestLedger). Response now includes dependencies (database, sorobanRpc) with status/latency, correlationId, checkedAt, and preserves the existing data field with lag information. ETag is computed from the deterministic parts only (dependencies + data) to maintain stability across requests.

• tests/indexerHealth.test.ts: Rewrote tests to use injectable deps pattern (following webhooksHealth.test.ts). Added 19 new test cases covering dependency probe status, overall status computation, correlationId, and error handling.

• tests/indexerHealthEtag.test.ts: Updated to use injectable deps pattern. All 12 ETag/conditional GET tests pass with the new response shape.

• README.md: Updated endpoint description to reflect the new dependency probe functionality.

Response shape

{
  "status": "ok" | "degraded" | "down",
  "correlationId": "<uuid>",
  "checkedAt": "<ISO-8601>",
  "dependencies": {
    "database":   { "status": "ok"|"down", "latencyMs": <n>, "error?": "..." },
    "sorobanRpc": { "status": "ok"|"down", "latencyMs": <n>, "error?": "..." }
  },
  "data": {
    "status":    "ok" | "degraded" | "down",
    "cursor":    <n>,
    "chainTip":  <n> | null,
    "lag":       <n> | null,
    "maxLag":    <n>
  }
}

• Overall status is "down" if any dependency is down or chain tip is unreachable, "degraded" if lag exceeds threshold, "ok" otherwise.
• HTTP status is always 200 (for uptime probe compatibility).

Closes #488

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