Silence the deprecation warnings our own code raises

Starlette renamed HTTP_422_UNPROCESSABLE_ENTITY to HTTP_422_UNPROCESSABLE_CONTENT
and now warns on the old spelling; the value is unchanged at 422, so the 20 call
sites in the routers move over with no wire-format or frontend impact.

Alembic >=1.16 supersedes version_path_separator with path_separator, which
governs both prepend_sys_path and version_locations, and warns when it is absent.

Together this takes pytest from 26 deprecation warnings to 6. The remainder are
slowapi calling asyncio.iscoroutinefunction; they are left visible deliberately,
with no filterwarnings entry, as a reminder to revisit once upstream ships a fix.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/app/routers/settings.py

@@ -83,7 +83,7 @@ async def create_child(
         )
         return _child_to_response(child)
     except ValidationError as exc:
-        raise HTTPException(status.HTTP_422_UNPROCESSABLE_ENTITY, str(exc)) from exc
+        raise HTTPException(status.HTTP_422_UNPROCESSABLE_CONTENT, str(exc)) from exc
 
 
 @child_router.get("/{child_id}", response_model=ChildResponse)
@@ -130,7 +130,7 @@ async def update_child(
         )
         return _child_to_response(child)
     except ValidationError as exc:
-        raise HTTPException(status.HTTP_422_UNPROCESSABLE_ENTITY, str(exc)) from exc
+        raise HTTPException(status.HTTP_422_UNPROCESSABLE_CONTENT, str(exc)) from exc
     except NotFoundError as exc:
         raise HTTPException(status.HTTP_404_NOT_FOUND, str(exc)) from exc
 
@@ -164,6 +164,6 @@ async def update_parent_settings(
         )
         return PersonalDataResponse.from_entity(updated)
     except ValidationError as exc:
-        raise HTTPException(status.HTTP_422_UNPROCESSABLE_ENTITY, str(exc)) from exc
+        raise HTTPException(status.HTTP_422_UNPROCESSABLE_CONTENT, str(exc)) from exc
     except NotFoundError as exc:
         raise HTTPException(status.HTTP_404_NOT_FOUND, str(exc)) from exc