fix: consolidate duplicate indexer integration tests

[wheval]

Summary

Consolidates duplicate indexer integration test suites into a single tests/ layout: worker lifecycle tests live in indexer-worker.test.ts, HTTP route tests stay in streams.test.ts, and DB-backed flows remain in stream-lifecycle.test.ts.

Purpose / Motivation

Three overlapping files exercised the same indexer → stream flows (src/__tests__/integration/streams.test.ts, tests/integration/streams.test.ts, and tests/integration/stream-lifecycle.test.ts). The split src/__tests__/ location also duplicated Vitest discovery paths and slowed CI.

Changes Made

• Moved worker-based indexer tests (including fee_config_updated and admin_transferred) from src/__tests__/integration/streams.test.ts to tests/integration/indexer-worker.test.ts.
• Removed duplicate POST-based lifecycle blocks from tests/integration/streams.test.ts; that file now covers claimable fallback, SSE contracts, and events pagination only.
• Deleted backend/src/__tests__/integration/streams.test.ts.
• Narrowed vitest.config.ts include to tests/** only.

How to Test

1. From backend/, run the full suite:

   npx vitest run

2. Run the consolidated integration files:

   npx vitest run tests/integration/indexer-worker.test.ts tests/integration/streams.test.ts tests/integration/stream-lifecycle.test.ts

3. Expected: all tests pass; no references to src/__tests__/ remain.

Breaking Changes

None.

Related Issues

Closes #548

Checklist

• Code builds successfully
• Tests added/updated
• No console errors
• Documentation updated (if needed)