fix(backend): extract shared indexer state id

[Bug-Hunter-X]

Overview

This PR extracts the hardcoded singleton IndexerState ID into one shared constant so all live call sites import the same value instead of duplicating the string in multiple files. It keeps the existing runtime behavior unchanged while making future updates safer.

Related Issue

Closes #641

Changes

🔧 Shared Indexer State Constant

• [ADD] backend/src/lib/indexer-state.ts
• Added a single exported INDEXER_STATE_ID constant for the IndexerState primary key.

🔄 Updated Call Sites

• [MODIFY] backend/src/workers/soroban-event-worker.ts
• [MODIFY] backend/src/services/indexerService.ts
• [MODIFY] backend/src/routes/health.routes.ts
• [MODIFY] backend/src/routes/v1/admin.routes.ts
• Replaced the hardcoded 'singleton' value with INDEXER_STATE_ID in the live backend call sites.
• Kept behavior unchanged.

Verification Results

Acceptance Criteria	Status
Constant moved to a shared module	✅
All live call sites import it	✅
No behavior change	✅
Backend build passes	✅

How to Test

# 1. Confirm you're on the branch
git branch --show-current

# 2. Verify the backend builds successfully
npm run build

# 3. Optional: inspect the refactor
git diff -- backend/src/lib/indexer-state.ts backend/src/workers/soroban-event-worker.ts backend/src/services/indexerService.ts backend/src/routes/health.routes.ts backend/src/routes/v1/admin.routes.ts

Screenshots

✅ Backend build passes

Run:

npm run build

[ogazboiz]

clean — closes #641 (INDEXER_STATE_ID extracted to one shared constant across 5 files). all CI green. merging!

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