Refactor: Continuous conversation, UI improvements, and build fixes

Merge branch 'main' of https://huggingface.co/spaces/colearn30/colearn30-ai-grading-agents into main

fix: resolve syntax error in openai_service.py

Update backend/app/services/openai_service.py

Update backend/app/routers/grading.py

Update backend/app/models/grading.py

feat: add continuous conversation, relax prompt constraints, and fix docker config

Update backend/app/models/grading.py

Update backend/app/routers/grading.py

Update backend/app/services/openai_service.py

Update backend/app/services/openai_service.py

Update backend/agents.yaml

Update backend/agents.yaml

Update backend/agents.yaml

Revert "chore: Remove duplicate agents.yaml from root, keep only in backend/"

chore: Remove duplicate agents.yaml from root, keep only in backend/

fix: Add agents.yaml to backend directory for HF Spaces deployment

fix: Improve agents.yaml path detection with detailed logging

fix: Auto-detect agents.yaml path for different environments

feat: Refactor assistant system with auto-matching and manual selection

Add missing password protection files

Add password protection system

fix: Add explicit extra_headers for assistants v2 on all API calls

FORCE: HF Spaces rebuild with API v2 fix - build 0049

debug: Use manual version number for HF deployment tracking

debug: Add commit hash to API title for version tracking

debug: Add logging to verify API v2 header initialization

fix: Add API v2 header to assistant_setup.py

fix: Update to OpenAI Assistants API v2

fix: Remove conflicting root route from backend main.py

fix: Resolve Python import paths for HF Spaces deployment

feat: Complete AI grading platform with multilingual support