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/auth.py

@@ -116,7 +116,7 @@ async def get_context(
     """Return workspace + children for the current user. Used by the dev harness."""
     pd = await pd_repo.get_by_id(current_user.personal_data_uuid)
     if pd is None:
-        raise HTTPException(status.HTTP_422_UNPROCESSABLE_ENTITY, "No parent profile found")
+        raise HTTPException(status.HTTP_422_UNPROCESSABLE_CONTENT, "No parent profile found")
     children = await child_repo.list_by_workspace(pd.workspace_id)
     return _UserContext(
         personal_data_uuid=str(pd.uuid),