fix(db): business-key unique constraints (M16-M19)

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>

src/Appointment/Entity/DateOverride.php

@@ -9,7 +9,7 @@ use Symfony\Component\Uid\Uuid;
 
 #[ORM\Entity(repositoryClass: DateOverrideRepository::class)]
 #[ORM\Table(name: 'date_overrides')]
-#[ORM\Index(columns: ['doctor_id', 'date'], name: 'idx_date_overrides_doctor_date')]
+#[ORM\UniqueConstraint(name: 'uniq_date_override_doctor_date', columns: ['doctor_id', 'date'])]
 class DateOverride
 {
     #[ORM\Id]