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/plan.py

@@ -36,7 +36,7 @@ async def propose_plan(
     except NotFoundError as exc:
         raise HTTPException(status.HTTP_404_NOT_FOUND, str(exc)) from exc
     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 PlanProposalResponse.from_domain(proposal)
 
 
@@ -100,7 +100,7 @@ async def sync_plan(
     except NotFoundError as exc:
         raise HTTPException(status.HTTP_404_NOT_FOUND, str(exc)) from exc
     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 PlanSyncResponse.from_domain(result)
 
 
@@ -147,5 +147,5 @@ async def sync_plan_with_wish(
     except NotFoundError as exc:
         raise HTTPException(status.HTTP_404_NOT_FOUND, str(exc)) from exc
     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 PlanSyncWithWishResponse.from_domain(result)