Wire a fourth source type through the retrieval, API, UI, and eval
dashboard layers:
- api/hybrid.py: add 'icd12' to SOURCE_TYPES so per-source retrieval
fans out to it alongside mtsamples, pubmed, icd11.
- api/main.py: add 'icd12' to the SourceType Literal so the JSON API
accepts it in source_types filters.
- api/templates/_results.html: amber chip treatment for icd12 hits.
- api/static/eval.js: amber colour + inclusion in the per-query source
mix chart on the /eval dashboard.
