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/preschool_planning_days.py

@@ -32,7 +32,7 @@ async def upsert_planned_days(
     try:
         entity = await service.upsert_planned(child, year, payload.planned_days)
     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
     return PreschoolPlanningDaysResponse.from_entity(entity)
 
 
@@ -48,7 +48,7 @@ async def record_taken_days(
     try:
         entity = await service.record_taken(child, year, payload.taken_days)
     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
     return PreschoolPlanningDaysResponse.from_entity(entity)