feat(webstatement): tambah model AccountBalance dan relasi balances pada Account

- Menambahkan model `AccountBalance` dengan fitur berikut:
  - Properti `fillable` meliputi: `account_number`, `period`, `actual_balance`, `cleared_balance`.
  - Relasi `belongsTo` dengan model `Account`.
  - Scope query untuk filter berdasarkan `account_number` (`scopeForAccount`) dan `period` (`scopeForPeriod`).
  - Fungsi statis `getBalance` untuk mendapatkan saldo berdasarkan `account_number` dan `period`.

- Menambahkan method berikut pada model `Account`:
  - Relasi `hasMany` dengan `AccountBalance` untuk mendapatkan semua saldo terkait.
  - Method `getBalanceForPeriod` untuk mendapatkan saldo pada periode tertentu.

- Membuat migrasi untuk tabel `account_balances` dengan spesifikasi berikut:
  - Kolom: `account_number`, `period` (format: YYYY-MM), `actual_balance` (decimal), `cleared_balance` (decimal), `timestamps`.
  - Konstrain unik untuk pasangan `account_number` dan `period`.
  - Indeks pada kolom `account_number`, `period`, dan `created_at`.

Signed-off-by: Daeng Deni Mardaeni <ddeni05@gmail.com>

app/Models/Account.php

@@ -34,4 +34,23 @@ class Account extends Model
     {
         return $this->belongsTo(Customer::class, 'customer_code', 'customer_code');
     }
+    
+    /**
+     * Get all balances for this account.
+     */
+    public function balances()
+    {
+        return $this->hasMany(AccountBalance::class, 'account_number', 'account_number');
+    }
+    
+    /**
+     * Get balance for a specific period.
+     * 
+     * @param string $period Format: YYYY-MM
+     * @return AccountBalance|null
+     */
+    public function getBalanceForPeriod($period)
+    {
+        return $this->balances()->where('period', $period)->first();
+    }
 }

app/Models/AccountBalance.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace Modules\Webstatement\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+
+class AccountBalance extends Model
+{
+    use HasFactory;
+
+    /**
+     * The attributes that are mass assignable.
+     */
+    protected $fillable = [
+        'account_number',
+        'period',
+        'actual_balance',
+        'cleared_balance',
+    ];
+
+    /**
+     * Get the account that owns the balance.
+     */
+    public function account()
+    {
+        return $this->belongsTo(Account::class, 'account_number', 'account_number');
+    }
+
+    /**
+     * Scope a query to filter by account number.
+     */
+    public function scopeForAccount($query, $accountNumber)
+    {
+        return $query->where('account_number', $accountNumber);
+    }
+
+    /**
+     * Scope a query to filter by period.
+     */
+    public function scopeForPeriod($query, $period)
+    {
+        return $query->where('period', $period);
+    }
+
+    /**
+     * Get balance for a specific account and period.
+     *
+     * @param string $accountNumber
+     * @param string $period Format: YYYY-MM
+     * @return AccountBalance|null
+     */
+    public static function getBalance($accountNumber, $period)
+    {
+        return self::where('account_number', $accountNumber)
+            ->where('period', $period)
+            ->first();
+    }
+}