feat(backend): persist plan scores on MealPlanItem

- Migration 0012 adds score (float) and components (jsonb) to meal_plan_item
- generate.py: populates score and components at create time
- schemas/MealPlanItemResponse: include score + components fields
- GET /api/meal-plans/{id}: returns persisted values instead of zeros

backend/alembic/versions/0012_meal_plan_item_scores.py

@@ -0,0 +1,22 @@
+"""Persist score and components on MealPlanItem."""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision = "0012"
+down_revision = "0011"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("meal_plan_item", sa.Column("score", sa.Float, nullable=True))
+    op.add_column(
+        "meal_plan_item",
+        sa.Column("components", sa.dialects.postgresql.JSONB, nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("meal_plan_item", "components")
+    op.drop_column("meal_plan_item", "score")