feat(CP-26): reference data management service

[ncamaa]

Summary

• Adds a new src/api/reference/ module with cached service layer and REST endpoints for all reference data tables (tax brackets, pension coefficients, CBS expense standards, system constants)
• Service functions fall back to the latest available year when the requested year has no data, and pension coefficients interpolate between the two nearest entries for birth years not in the table
• In-memory cache (load once, cache forever) keeps repeated reads fast since reference data only changes annually

Endpoints

Method	Path	Description
GET	/api/reference/tax-brackets?year=2025	Tax brackets for a year
GET	/api/reference/pension-coefficients?birthYear=1985&gender=MALE	Pension coefficient (interpolates)
GET	/api/reference/cbs-expenses?householdSize=5	CBS per-capita monthly expense
GET	/api/reference/constants?year=2025	System constants as key-value map
GET	/api/reference/all?year=2025	Bundled reference data for the engine

Test plan

• 16 unit tests covering all service methods, fallback logic, interpolation, caching, and null cases
• pnpm test passes (227 tests)
• pnpm build passes (tsc + tsup)
• Curl verification against running dev server with seeded data