Removes DEFAULT_MODEL in favour of a force_model setting configurable
via the admin UI. When set, every proxy request's model field is
overridden, preventing uncoordinated model switches during lab sessions.
Updates schemas, admin API, all three proxy endpoints, frontend,
init_db, and docs (README, DOCKERHUB, KURZANLEITUNG).
Add a second usage log line after each proxy response with actual ↑prompt ↓completion
token counts from Ollama (prompt_eval_count/eval_count for native endpoints,
usage object for OpenAI endpoint). Also adds KURZANLEITUNG.md for students and
colleagues covering API access, model selection, Python examples, opencode setup,
and quota/admin information.
