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.

backend/routes/meta.py

@@ -12,7 +12,7 @@ router = APIRouter(prefix="/api/meta", tags=["meta"])
 async def get_constants() -> ConstantsResponse:
     """Get shared constants for frontend/backend synchronization."""
     return ConstantsResponse(
-        permissions=[p.value for p in Permission],
+        permissions=list(Permission),
         roles=[ROLE_ADMIN, ROLE_REGULAR],
-        invite_statuses=[s.value for s in InviteStatus],
+        invite_statuses=list(InviteStatus),
     )