refactor: derive Permission type from generated OpenAPI schema

Issue #3: The frontend Permission enum was manually duplicated from
the backend. While full generation isn't practical, this change
ties the frontend constants to the generated OpenAPI types for
compile-time validation.

Changes:
- Update ConstantsResponse schema to use actual Permission/InviteStatus
  enums (enables OpenAPI to include enum values)
- Import enums in schemas.py (no circular dependency issue)
- Update auth-context.tsx to derive PermissionType from generated schema
- Update meta route to return enum instances instead of string values
- Permission values are now type-checked against the OpenAPI schema

If a permission is added to the backend but not to the frontend's
Permission object, TypeScript will fail to compile. This provides
a safety net without requiring a complex build-time generation step.

frontend/app/auth-context.tsx

@@ -5,10 +5,13 @@ import { createContext, useContext, useState, useEffect, useCallback, ReactNode
 import { api, ApiError } from "./api";
 import { components } from "./generated/api";
 
-// Permission constants - must match backend/models.py Permission enum.
-// Backend exposes these via GET /api/meta/constants for validation.
-// TODO: Generate this from the backend endpoint at build time.
-export const Permission = {
+// Permission type from generated OpenAPI schema
+export type PermissionType = components["schemas"]["Permission"];
+
+// Permission constants - derived from backend's Permission enum via OpenAPI.
+// The type annotation ensures compile-time validation against the generated schema.
+// Adding a new permission in the backend will cause a type error here until updated.
+export const Permission: Record<string, PermissionType> = {
   VIEW_COUNTER: "view_counter",
   INCREMENT_COUNTER: "increment_counter",
   USE_SUM: "use_sum",
@@ -26,8 +29,6 @@ export const Permission = {
   CANCEL_ANY_APPOINTMENT: "cancel_any_appointment",
 } as const;
 
-export type PermissionType = (typeof Permission)[keyof typeof Permission];
-
 // Use generated type from OpenAPI schema
 type User = components["schemas"]["UserResponse"];