fix: add stream error handling + startup diagnostics, bump CACHEBUST

feat: overhaul model routing — GLM-5.1 primary, strict 4-model fallback chain, purge all stale refs

feat: Add full multi-turn conversation memory and context rewriting

fix(backend): bust docker cache + fix async streaming generator to not block event loop

feat: implement ChatGPT style UI, streaming generation, math/code blocks, citations & multi-model fallback

feat: FastAPI backend complete