Consolidate savings-rate math into CashflowSummaryService

The net and savings-rate derivation was byte-identical in
DashboardController::calculateCashflowSummary and
CashflowAnalyticsController::cashflowSummaryFromTransactions, so the two
screens could silently diverge if one formula were ever edited.

Extract a single CashflowSummaryService::summarize(income, expense) that
owns the net and savings-rate math, and have both call sites consume it.
Behaviour-preserving: each controller still computes income/expense its
own way (single-currency SQL on the dashboard, multi-currency in-memory
on the analytics endpoint) and only the shared derivation moves.

app/Http/Controllers/Api/CashflowAnalyticsController.php

@@ -8,6 +8,7 @@ use App\Http\Controllers\Api\Concerns\ConvertsTransactionCurrency;
 use App\Http\Controllers\Controller;
 use App\Models\Category;
 use App\Models\Transaction;
+use App\Services\CashflowSummaryService;
 use App\Services\CategoryTree;
 use App\Services\ExchangeRateService;
 use App\Services\PeriodComparator;
@@ -208,14 +209,8 @@ class CashflowAnalyticsController extends Controller
         $savings = $this->sumOutflowTransactions($transactions, $userCurrency, CategoryType::Savings);
         $investments = $this->sumOutflowTransactions($transactions, $userCurrency, CategoryType::Investment);
 
-        $net = $income - $expense;
-        $savingsRate = $income > 0 ? round((($income - $expense) / $income) * 100, 1) : 0;
-
         return [
-            'income' => $income,
-            'expense' => $expense,
-            'net' => $net,
-            'savings_rate' => $savingsRate,
+            ...CashflowSummaryService::summarize($income, $expense),
             'savings' => $savings,
             'investments' => $investments,
         ];

app/Http/Controllers/DashboardController.php

@@ -6,6 +6,7 @@ use App\Enums\CategoryType;
 use App\Models\Account;
 use App\Models\Transaction;
 use App\Services\AccountMetricsService;
+use App\Services\CashflowSummaryService;
 use App\Services\CategoryTree;
 use App\Services\PeriodComparator;
 use Carbon\Carbon;
@@ -135,15 +136,7 @@ class DashboardController extends Controller
         $income = max(0, $this->getTransactionSum($userId, $from, $to, CategoryType::Income));
         $expense = max(0, -$this->getTransactionSum($userId, $from, $to, CategoryType::Expense));
 
-        $net = $income - $expense;
-        $savingsRate = $income > 0 ? round((($income - $expense) / $income) * 100, 1) : 0;
-
-        return [
-            'income' => $income,
-            'expense' => $expense,
-            'net' => $net,
-            'savings_rate' => $savingsRate,
-        ];
+        return CashflowSummaryService::summarize($income, $expense);
     }
 
     private function getTransactionSum(string $userId, Carbon $from, Carbon $to, CategoryType $type): int

app/Services/CashflowSummaryService.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Services;
+
+class CashflowSummaryService
+{
+    /**
+     * Derive the shared cashflow summary from already-clamped income and
+     * expense totals (both non-negative, in minor units). Kept in one place so
+     * the net and savings-rate math can never drift between the dashboard and
+     * the cashflow analytics endpoints.
+     *
+     * @return array{income: int, expense: int, net: int, savings_rate: float|int}
+     */
+    public static function summarize(int $income, int $expense): array
+    {
+        return [
+            'income' => $income,
+            'expense' => $expense,
+            'net' => $income - $expense,
+            'savings_rate' => $income > 0 ? round((($income - $expense) / $income) * 100, 1) : 0,
+        ];
+    }
+}

tests/Unit/Services/CashflowSummaryServiceTest.php

@@ -0,0 +1,33 @@
+<?php
+
+use App\Services\CashflowSummaryService;
+
+it('derives net and savings rate from income and expense', function () {
+    expect(CashflowSummaryService::summarize(100000, 40000))->toBe([
+        'income' => 100000,
+        'expense' => 40000,
+        'net' => 60000,
+        'savings_rate' => 60.0,
+    ]);
+});
+
+it('returns a negative net when expense exceeds income', function () {
+    expect(CashflowSummaryService::summarize(0, 10000))->toBe([
+        'income' => 0,
+        'expense' => 10000,
+        'net' => -10000,
+        'savings_rate' => 0,
+    ]);
+});
+
+it('avoids division by zero when income is zero', function () {
+    $summary = CashflowSummaryService::summarize(0, 0);
+
+    expect($summary['savings_rate'])->toBe(0)
+        ->and($summary['net'])->toBe(0);
+});
+
+it('rounds the savings rate to one decimal place', function () {
+    // (100000 - 33333) / 100000 * 100 = 66.667 -> 66.7
+    expect(CashflowSummaryService::summarize(100000, 33333)['savings_rate'])->toBe(66.7);
+});