Functional: Pricing service implementation

Implement the Pricing service per reference spec.

**Spec**
- See: https://github.com/chrislyons-dev/bond-math/blob/main/docs/reference/pricing.md

**Acceptance**
- [ ] Endpoints implemented per spec
- [ ] Input validation and error model
- [ ] Integration tests with valuation and day-count
- [ ] OpenAPI (or typed client) published
- [ ] CI contract tests