docs: add architecture and implementation documentation for SSE integ…

[muhsar27]

Closes #1300

Description

Corrects the SSE architecture diagrams and documentation in SSE_ARCHITECTURE.md and SSE_IMPLEMENTATION.md. The previous diagrams and prose incorrectly showed HTTP API controllers (e.g., Stream Controller) invoking sseService.broadcast() synchronously upon receiving user actions. In reality, all stream/protocol SSE events originate asynchronously from the background indexer worker (SorobanEventWorker) only after transaction confirmation on the Stellar ledger, with computed runway alerts emitted by StreamRunwayWorker.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test addition or update

Related Issues

Closes #

Changes Made

• Updated the System Flow ASCII diagram in backend/docs/SSE_ARCHITECTURE.md to show event flow originating from Stellar/Soroban ledger executions to SorobanEventWorker (Indexer), which persists state and dispatches sseService.broadcastToStream/Admin().
• Added an explanatory note clarifying that HTTP API controllers (stream.controller.ts, sse.controller.ts) do not trigger broadcasts directly and that SSE events are delayed/indexer-driven after on-chain confirmation.
• Updated the Horizontal Scaling with Redis flow description to indicate that the worker instance processing on-chain events publishes to Redis.
• Added sseService.broadcastToAdmin() to the Event Broadcasting Logic section alongside broadcastToStream() and broadcastToUser().
• Updated publisher reference comment in backend/docs/SSE_IMPLEMENTATION.md to reflect SorobanEventWorker / indexer instead of stream controller.

Testing

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Test Steps

1. Cross-referenced all sseService.broadcast* call sites across backend/src/ (soroban-event-worker.ts, stream-runway-worker.ts, sse.service.ts) against the updated documentation.
2. Verified markdown syntax and diagram rendering in backend/docs/SSE_ARCHITECTURE.md and backend/docs/SSE_IMPLEMENTATION.md.

Breaking Changes

None.

Screenshots/Demo

N/A (Documentation update).

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Updated Postman/Hoppscotch API collections if routes changed
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• I have checked for breaking changes and documented them if applicable

Additional Notes

This update prevents confusion when debugging stream lifecycle events (e.g., pause, withdraw, top-up, cancel) where SSE events appear with ledger latency rather than synchronously upon HTTP API invocation.