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/planned_days.py

@@ -30,10 +30,10 @@ def _to_http_error(exc: ValidationError) -> HTTPException:
     """
     if exc.message_key:
         return HTTPException(
-            status.HTTP_422_UNPROCESSABLE_ENTITY,
+            status.HTTP_422_UNPROCESSABLE_CONTENT,
             detail={"message_key": exc.message_key, "data": exc.data},
         )
-    return HTTPException(status.HTTP_422_UNPROCESSABLE_ENTITY, detail=str(exc))
+    return HTTPException(status.HTTP_422_UNPROCESSABLE_CONTENT, detail=str(exc))
 
 
 @router.post("", response_model=PlannedLeaveDayResponse, status_code=status.HTTP_201_CREATED)