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/calendar.py

@@ -31,5 +31,5 @@ async def get_calendar_days(
             workspace_id=workspace_id, start_year=start_year, years=years
         )
     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 CalendarDaysResponse.from_domain(window)