backend: add per-clinic staff specialty (Care Team -> patient sheet)

New staff_profile table (org + user, unique) holding a clinician's clinical
specialty. GET /api/staff and /api/staff/providers now include specialty; new
PATCH /api/staff/:userId (member:update) upserts it. Migration 0023.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/db/schema/index.ts

@@ -16,3 +16,4 @@ export * from "./ai-chat.js";
 export * from "./org-ai-policy.js";
 export * from "./attachments.js";
 export * from "./integrations.js";
+export * from "./staff-profile.js";