docs: clarify indexer ownership and naming (indexerService.ts / soroban-indexer.service.ts)

[OpensrcLord]

Fixes #1096

Summary

Documents which indexer-related file is the source of truth and which is legacy/being phased out, so new contributors know where to start when debugging indexing.

Changes

• backend/src/services/indexerService.ts: added a module-level JSDoc clarifying it is not an indexer — it is the admin control-plane helper (getIndexerStatus / resetIndexer / replayFromLedger) for the source-of-truth worker.
• backend/src/services/soroban-indexer.service.ts: added a module-level JSDoc marking it as the legacy indexer being phased out; SorobanEventWorker is the source of truth.
• docs/ARCHITECTURE.md: added an "Indexer Ownership & Naming" section with a file/role/status table, key debugging pointers, and the naming-convention plan.

Notes

• The double-indexer race itself (issue [Backend] Two indexers run concurrently - soroban-indexer.service + soroban-event-worker race on the same rows and double-count withdrawnAmount #801) is out of scope here, as specified in the issue.
• Renaming files is deferred per the issue's acceptance criteria (once the functional consolidation (tracked separately) lands); the intended rename (indexerService.ts → indexer.service.ts, kebab-case + .service.ts) is documented in the comments and ARCHITECTURE.md.