Improve FoodItem uniqueness and add name constraint

Refactored add_meal routes to check for existing FoodItems by name or barcode and improved form handling. Made barcode optional in FoodItemForm. Added a unique constraint on (name, owner_id) for FoodItem in both the model and database migrations, while retaining the (barcode, owner_id) constraint. Updated FoodItem relationship to cascade deletes.

migrations/versions/f5fbbe915d51_.py

@@ -0,0 +1,32 @@
+"""empty message
+
+Revision ID: f5fbbe915d51
+Revises: 99f86450e4af
+Create Date: 2025-08-11 12:37:02.407643
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'f5fbbe915d51'
+down_revision = '99f86450e4af'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table('food_item', schema=None) as batch_op:
+        batch_op.create_unique_constraint('barcode_owner_key', ['barcode', 'owner_id'])
+
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table('food_item', schema=None) as batch_op:
+        batch_op.drop_constraint('barcode_owner_key', type_='unique')
+
+    # ### end Alembic commands ###