Pet interface: added sizeCategory and coatType (nullable strings).
Service interface: added defaultBufferMinutes.
Appointment interface: added bufferMinutes.
These fields are referenced by Book.tsx, cascade.ts, buffer.ts, appointment
routes, and other type-annotated consuming code. Without them, any file that
imports these interfaces and accesses the fields causes a TypeScript error.
cc @cpfarhood
Co-Authored-By: Flea Flicker <noreply@paperclip.ing>