[Chore] Remove duplicate/aspirational SSE status files from backend root

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

backend/SSE_README.md is an SSE Implementation Summary status dump and backend/PRODUCTION_CHECKLIST.md is an SSE-only checklist of mostly unimplemented snippets (k8s manifests, nginx, prom-client) referencing non-existent prod domains. The canonical, maintained SSE docs already live at backend/docs/SSE_IMPLEMENTATION.md and backend/docs/SSE_ARCHITECTURE.md, so these two root files are stale duplicates.

Acceptance criteria

• Remove backend/SSE_README.md and backend/PRODUCTION_CHECKLIST.md, or fold any still-accurate content into backend/docs/SSE_IMPLEMENTATION.md / SSE_ARCHITECTURE.md
• Ensure README's API/SSE section still points only to docs that exist

Files to touch

• backend/SSE_README.md
• backend/PRODUCTION_CHECKLIST.md
• backend/docs/SSE_IMPLEMENTATION.md
• backend/docs/SSE_ARCHITECTURE.md

Out of scope

• Rewriting the SSE implementation itself
• test-sse-client.html