feat(logging): wire structured logger into Stellar indexer

[Jaydearcadian]

Closes #423

Summary

• Adds lib/logger.ts around Pino with LOG_LEVEL support, info default, production JSON, and development pino-pretty output.
• Converts all runtime logging in lib/stellar/indexer.ts to logger.child({ module: "indexer" }).
• Preserves retry, cursor, Horizon stream, callback, and error behavior with structured fields.
• Leaves the other Stellar files untouched, per issue scope.

Verification

• npm ci passed.
• npm test -- lib/__tests__/logger.test.ts lib/stellar/__tests__/indexer.test.ts passed: 9/9 tests.
• Console audit passed: no console. calls remain in lib/stellar/indexer.ts.
• git diff --check passed.
• Full repository typecheck/build blockers are pre-existing and outside Wire up the already-installed structured logger instead of ad-hoc console.log #423.

Scope

This PR intentionally does not migrate client.ts, resilient-stellar-client.ts, or indexer-persistent.ts.