Add unique constraints (one migration, no dup data in either DB):
- users.email, users.national_code (M16) — NULLs still allowed.
- payments.gateway_token (M17).
- date_overrides (doctor_id, date) (M18) — was a non-unique index.
- financial_breakdowns (payment_id, source) (M19) — anti double-accounting.
Regression: tests/Database/UniqueConstraintsTest (4 duplicate-insert cases).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
