feat: full appointment feature (CRUD + Appointments tab)

Builds the complete appointment feature end-to-end, mirroring the medication
feature structure. The model existed but was orphaned dead code (no handlers,
routes, repo, or frontend) — this builds the whole stack.

Backend:
- models/appointment.rs: AppointmentData (deserializes the data blob, camelCase
  keys), AppointmentResponse (flat snake_case + From<Appointment>), Create/Update
  request structs, AppointmentRepository (create, find_by_user_filtered with
  optional status filter, find/update/delete by appointment_id). Mirrors the
  medication serialization pattern. Module added to models/mod.rs.
- handlers/appointments.rs: full CRUD (create/list/get/update/delete) returning
  AppointmentResponse. Routes wired in app.rs
  (POST/GET /api/appointments, GET/POST /:id, POST /:id/delete).
- 3 unit tests (AppointmentData deser, default status, response flatten).

Frontend:
- Appointment/Create/Update types; getAppointments/getAppointment/create/
  update/deleteAppointment in api.ts; useAppointmentStore.
- AppointmentsManager component (list with type+status chips, create/edit
  dialogs, delete confirm, empty state) as a 5th Dashboard tab.

Verified: backend fmt/clippy 0 warnings, 26 tests pass; frontend build clean,
20 tests pass.

backend/src/models/mod.rs

@@ -1,3 +1,4 @@
+pub mod appointment;
 pub mod audit_log;
 pub mod family;
 pub mod health_data;