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/Settlement/Entity/FinancialBreakdown.php

@@ -12,6 +12,7 @@ use Symfony\Component\Uid\Uuid;
 #[ORM\Table(name: 'financial_breakdowns')]
 #[ORM\Index(columns: ['representation_id', 'created_at'], name: 'idx_breakdown_rep_date')]
 #[ORM\Index(columns: ['source', 'created_at'], name: 'idx_breakdown_source_date')]
+#[ORM\UniqueConstraint(name: 'uniq_breakdown_payment_source', columns: ['payment_id', 'source'])]
 class FinancialBreakdown
 {
     public const SOURCE_APPOINTMENT  = 'appointment';