refactor: remove dead ZeroGPU shim (Docker Spaces don't support ZeroGPU)

fix(progress): run sync adapter.generate in a thread so SSE events stream during generation

feat(progress): wrap /api/generate/dialog in session; emit per-turn events

feat(progress): /api/progress SSE endpoint; wrap /api/generate in session

feat(dialog): /api/generate/dialog endpoint + per-turn dispatcher with seed reuse

feat(api): adapter generate returns seed_used; expose X-Seed-Used header

chore(api,web): include validation message and detail in /api/generate error response

feat(api,models): FastAPI app with /api/health, /api/models, activate, /api/generate + chatterbox-en adapter