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/fetch_data.py

@@ -82,7 +82,7 @@ async def save_manual_entry(
             source=payload.source,
         )
     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 ParentalLeaveFetchResponse.from_entity(fetch)
 
 
@@ -170,7 +170,7 @@ async def start_fetch(
         children = await child_repo.list_by_workspace(workspace_id)
         if not children:
             raise HTTPException(
-                status.HTTP_422_UNPROCESSABLE_ENTITY,
+                status.HTTP_422_UNPROCESSABLE_CONTENT,
                 "No child in workspace — add one before fetching.",
             )
         child_id = children[0].id