E-Commerce Platform — Schema Evolution Migration

Problem Description

An e-commerce company's SQL Server database has been running in production for two years and needs several schema changes as part of a new feature release. The engineering team deploys schema migrations by running SQL scripts directly against production — and occasionally against staging environments that may have already received an earlier version of the same migration. A script that crashes on re-run (because the column already exists, or the constraint is already there) causes downtime incidents.

The team needs a migration script that can be safely run multiple times: if the changes are already applied, running the script again must be a no-op. If they're not applied, the script applies them correctly. The deployment pipeline cannot guarantee a script runs exactly once.

The required schema changes are:

1. Add a IsVerified boolean column to the Customer table (default false)
2. Add a DiscountPct decimal column to the Product table (allow NULL — some products have no discount)
3. Add a new OrderTag child table that links tags to orders — composite key of CustomerNo + OrderNo + TagNo, with a foreign key to the Order table
4. Add a unique constraint on Customer.Email
5. Add a non-clustered index on Order.CreatedAt
6. Add a new custom type TagLabel (VARCHAR-based) if it doesn't already exist

The existing schema uses a custom type system (custom types like _Bool, _Money, CustomerNo, OrderNo, TagNo already exist). You can assume the meta-infrastructure functions may or may not exist yet and should be created as part of this migration if needed.

Output Specification

Produce two files:

1. meta_functions.sql — helper functions for checking whether database objects already exist, so the migration script can use them
2. migration.sql — the migration script itself that applies each of the 6 changes safely regardless of whether they have already been applied

Both files should be self-contained SQL. The migration.sql should include at the top: -- Run this script on any environment, any number of times. It is idempotent.