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/CustomFieldValue.php

@@ -24,7 +24,7 @@ class CustomFieldValue extends Model
         'defaultAnswer',
     ];
 
-    public function setTimeAnswerAttribute($value)
+    public function setTimeAnswerAttribute(mixed $value): void
     {
         if ($value && $value != null) {
             $this->attributes['time_answer'] = date('H:i:s', strtotime($value));