feat(diagnoses): ICD-10 search + attach diagnoses to encounters

[rmsb-art]

Implements standardized diagnosis coding for encounters in LumenHealth EHR, enabling doctors to attach ICD-10 codes instead of free-text diagnoses.

Backend

• Added DiagnosisModel for storing ICD-10 codes and descriptions.
• Implemented scripts/seed-icd10-lite.ts to populate MongoDB with ~1,000 common primary care ICD-10 codes (idempotent).
• Added MongoDB text indexes to support fuzzy diagnosis search.
• Implemented GET /api/v1/diagnoses/search?q= for fast code lookup (<100ms).
• Implemented POST /api/v1/encounters/:encounterId/diagnoses to attach diagnoses to an encounter (uses hardcoded encounterId for now).

Frontend

• Built async debounced Combobox (Tags Input) using shadcn/ui.
• Fetches diagnoses via the search endpoint with strict debounce to prevent API spam.
• Selected diagnoses render as removable badges
  e.g. [B50.9 - Malaria ✕].

Notes

• Seed script can be safely re-run without duplicating codes.
• UI is built independently of Issue The Encounter State Machine & Security Locks #113 using a temporary encounter ID.

Acceptance

• ICD-10 dataset seeds successfully and idempotently.
• Search endpoint returns results in <100ms.
• Frontend debounces requests before calling the API.

closes #118

[inteee]

Approved. The changes look good and align with the project requirements. Thanks for the contribution.