Add return types, typed parameters, and PHPDoc to all model methods

Modernize all 16 models with missing type declarations:
- Return types on ~87 methods (string, bool, void, array, mixed, etc.)
- Typed parameters where missing
- PHPDoc blocks on non-obvious methods explaining their purpose

Models updated: Invoice, Estimate, Payment, User, Company, Customer,
RecurringInvoice, Setting, CompanySetting, FileDisk, Transaction,
EmailLog, ExchangeRateLog, PaymentMethod, CustomField, CustomFieldValue.

app/Models/Customer.php

@@ -119,7 +119,7 @@ class Customer extends Authenticatable implements HasMedia
         return $this->hasOne(Address::class)->where('type', Address::SHIPPING_TYPE);
     }
 
-    public function sendPasswordResetNotification($token)
+    public function sendPasswordResetNotification(mixed $token): void
     {
         $this->notify(new CustomerMailResetPasswordNotification($token));
     }