BE-046 Indexer module scaffold

[Kaylahray]

Description

Create the indexer module shell and register its service.

User Story

As a contributor,
I want to isolate the indexer into its own module,
so that chain polling logic has a stable backend boundary.

Requirements and Context

• File: backend/src/indexer/indexer.module.ts
• Register IndexerService

Suggested Implementation

// IN FILE: src/indexer/indexer.module.ts
// 1. Create the Nest indexer module.
// 2. Register the IndexerService.

Acceptance Criteria

• IndexerModule imports cleanly into AppModule.
• The service resolves through DI.

Submission Guidelines

• Branch: backend/be-046-indexer-module
• Depends on: BE-003, BE-013, BE-014
• PR: feat(backend): scaffold indexer module