fix(sku): add DB CHECK backstop rejecting empty SKU ID

[wminckler]

The code-level guard in db::sku::create/replace rejects empty ids, but only on those two paths. The machine_skus PRIMARY KEY blocks NULL yet accepts the empty string, so an empty id could still reach the table via any unvalidated write path.

Add a CHECK (id <> '') constraint so the database enforces the invariant regardless of caller. Any pre-existing empty-id SKU is preserved: it is reassigned a generated id and referencing machines are repointed before the constraint is applied, so the migration is non-destructive.

Verified via the sqlx_test harness, which rebuilds the migrated template DB from scratch and runs test_sku_create_rejects_empty_id green.

Description

Type of Change

• Add - New feature or capability
• Change - Changes in existing functionality
• Fix - Bug fixes
• Remove - Removed features or deprecated functionality
• Internal - Internal changes (refactoring, tests, docs, etc.)

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[wminckler]

draft because I want to run the migration manually in my dev-env

[wminckler]

draft because I want to run the migration manually in my dev-env

done. also verified that the check works by running the migration in an env that doesn't have the code fix in #2253