fix: atomic SQL swap for meal move endpoint

Replace two-step ORM update with single UPDATE ... CASE statement.
Eliminates IntegrityError from SQLAlchemy flush order violating the
unique constraint (meal_plan_id, day_of_week, meal_type).

backend/app/api/meals.py

@@ -2,6 +2,7 @@ from fastapi import APIRouter, Depends, HTTPException, Query
 from fastapi.responses import HTMLResponse
 from html import escape as _html_escape
 from pydantic import BaseModel, Field
+from sqlalchemy import text
 from sqlalchemy.orm import Session, joinedload
 from app.database import get_db
 from app.models import (
@@ -351,9 +352,35 @@ def move_meal_item(
     )
 
     if conflict:
-        # swap day/type
-        conflict.day_of_week = item.day_of_week
-        conflict.meal_type = item.meal_type
+        # Atomic swap via single SQL CASE statement — no intermediate
+        # unique-constraint violations.
+        db.execute(
+            text(
+                """
+                UPDATE meal_plan_item
+                SET day_of_week = CASE
+                    WHEN id = :item_id THEN :new_day
+                    WHEN id = :conflict_id THEN :old_day
+                END,
+                meal_type = CASE
+                    WHEN id = :item_id THEN :new_type
+                    WHEN id = :conflict_id THEN :old_type
+                END
+                WHERE id IN (:item_id, :conflict_id)
+                """
+            ),
+            {
+                "item_id": str(item_id),
+                "conflict_id": str(conflict.id),
+                "new_day": new_day_of_week,
+                "new_type": new_meal_type,
+                "old_day": item.day_of_week,
+                "old_type": item.meal_type,
+            },
+        )
+        db.commit()
+        db.refresh(item)
+        return {"message": "Meal swapped", "item": item}
 
     item.day_of_week = new_day_of_week
     item.meal_type = new_meal_type