WS/SSE: push model for live stream and settlement status to dashboard clients (backend)

[greatest0fallt1me]

WebSocket or SSE channel for stream:updated and settle:finished with auth

    ## Description

    The **StreamPay** dashboard (see `StreamPay-Frontend`) needs **low-latency** updates. Add

backend-first WebSocket or SSE with JWT or session auth, per-stream
subscribe, and backpressure so one tab cannot crush the service. This issue is
not the UI/UX design; that is tracked separately in the design backlog.
Complements existing BullMQ/queue and indexer events.

    ## Requirements and context

    - **Auth** to subscribe to own streams only; `403` on id guessing.

• Reconnection token pattern; Last-Event-Id for SSE.

• Message size caps; ping/pong to detect dead clients.

• Load test: N concurrent clients; memory stability.

• Security: no PII in event payload beyond what the UI already has.

    ## Suggested execution
  
    1. `git checkout -b feature/ws-sse-stream-status`
  

2. Choose WS vs SSE per deploy constraints; document.

3. Tests: integration with fake client; auth negative tests.

4. PR with protocol doc and example curl or websocat for QA.

5. Figma handoff: link event names only (separate project).

    ## Test and commit
   

• Run the full test suite; add or update tests until the agreed coverage bar is met.
• Cover edge cases listed in this issue; document any intentional exclusions with brief rationale in the PR.
• Include relevant test output (e.g. test runner summary) or a link to a passing CI run in the pull request.
• Add security notes for auth, keys, PII, chain settlement, or money movement (assumptions verified, out-of-scope items).

Example commit message

feat(api): add authenticated WS or SSE for live stream and settlement status updates

Guidelines

• Target: at least 95% coverage on new or meaningfully changed code (per the repo’s standard tooling).
• Documentation: update contributor-facing or API documentation where a reviewer would be blocked without it.
• Timeframe: 96 hours to ready-for-review (surface blockers early).