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/Payment/Entity/Payment.php

@@ -55,7 +55,7 @@ class Payment
     #[ORM\Column(type: 'string', length: 30)]
     private string $type;
 
-    #[ORM\Column(name: 'gateway_token', type: 'string', length: 255, nullable: true)]
+    #[ORM\Column(name: 'gateway_token', type: 'string', length: 255, nullable: true, unique: true)]
     private ?string $gatewayToken = null;
 
     #[ORM\Column(name: 'reference_id', type: 'string', length: 255, nullable: true, unique: true)]