test: cover auth migration constraints

[kpassito]

Summary

• Add focused Vitest coverage for the auth template migration SQL.
• Execute the real migration in an in-memory SQLite database.
• Cover auth_users username/email constraints, cross-field overlap trigger behavior, and auth_sessions foreign key / unique token constraints.

/claim #71

Verification

• ./node_modules/.bin/vitest run templates/auth/src/migration.test.ts
• ./node_modules/.bin/prettier --check templates/auth/src/migration.test.ts
• git diff --check

Note: full repository typecheck/full test suite still has existing unrelated failures in current main (RLS expectations / TS errors outside this file).