[FEAT] Backend: Implement Soroban Event Indexer #200

[CMI-James]

Description

Implement a Soroban polling indexer service in the backend to sync stream lifecycle events into Postgres for fast dashboard reads.

Closes #200

Changes proposed

What were you told to do?

I was asked to:

• set up polling with Stellar RPC,
• index StreamCreatedEvent into database records,
• update stream status/data for StreamCancelledEvent and TokensWithdrawnEvent.

What did I do?

Added Soroban event polling indexer service

Created backend/src/services/soroban-indexer.service.ts:

• polls Soroban RPC getEvents on interval,
• filters by configured stream contract (STREAM_CONTRACT_ID),
• tracks last indexed ledger in memory for incremental polling,
• parses stream events from contract topics/payload.

Stream + event database synchronization logic

Implemented handlers for:

• stream_created -> upsert users and create/update Stream row,
• stream_cancelled -> mark Stream.isActive = false,
• tokens_withdrawn -> increment withdrawnAmount and update active status.

All indexed events are persisted to StreamEvent, with duplicate guard by (streamId, eventType, transactionHash, ledgerSequence) lookup.

Startup wiring

Updated backend/src/index.ts to start the indexer after DB connection succeeds.

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• This PR does not contain plagiarized content.
• The title and description of the PR is clear and explains the approach.
• My commit messages styles matches our requested structure.
• My code additions will fail neither code linting checks nor unit test.
• I am only making changes to files I was requested to.

Screenshots / Testing Evidence

Attempted validation:

npm run build --workspace=backend

Build fails on pre-existing backend TypeScript issues unrelated to this PR (existing type/config errors in multiple backend files).