feat: calculation rules engine for decoupled cost attribution and chargeability

Introduces an admin-configurable rules engine that determines per-day cost
attribution (CHARGE/ZERO/REDUCE) and chargeability reporting (COUNT/SKIP)
for absence types (sick, vacation, public holiday). Includes shared types,
Zod schemas, Prisma model, rule matching with specificity scoring, default
rules, calculator integration, CRUD API router, seed data, chargeability
report integration, and admin UI.

283/283 engine tests, 209/209 API tests, 0 TS errors.

Co-Authored-By: claude-flow <ruv@ruv.net>

packages/engine/src/index.ts

@@ -6,3 +6,4 @@ export * from "./shift/index.js";
 export * from "./vacation/utils.js";
 export * from "./sah/index.js";
 export * from "./chargeability/index.js";
+export * from "./rules/index.js";