Merge branch 'worktree-fix-deprecations-backend-plan'

Rename Starlette HTTP_422_UNPROCESSABLE_ENTITY to HTTP_422_UNPROCESSABLE_CONTENT
across routers and switch alembic.ini to path_separator.

Conflicts: main had already deleted planned_day_exclusions.py and the write
endpoints in planned_days.py, so main's side was kept there. The rename was
also applied to day_assertion.py and the newer plan.py handler that main
added after the branch point.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T4wjgbVL4HAC5Rdaw75VbV

backend/app/routers/day_assertion.py

@@ -45,10 +45,10 @@ async def assert_day(
     except ValidationError as exc:
         if exc.message_key:
             raise HTTPException(
-                status.HTTP_422_UNPROCESSABLE_ENTITY,
+                status.HTTP_422_UNPROCESSABLE_CONTENT,
                 detail={"message_key": exc.message_key, "data": exc.data},
             ) from exc
-        raise HTTPException(status.HTTP_422_UNPROCESSABLE_ENTITY, str(exc)) from exc
+        raise HTTPException(status.HTTP_422_UNPROCESSABLE_CONTENT, str(exc)) from exc
     return DayAssertionResponse.from_entity(entity)