fix(cashflow): only count sign-matching transactions in Sankey category breakdown (#232)

## Problem

When an income category contained both incoming and outgoing
transactions (e.g. an \"Income from Rents\" category that also holds
property-related expense payments), the Sankey endpoint was summing
**all** amounts together and applying `abs()` to the net result.

Real example reported:

| Date | Description | Amount |
|------|-------------|--------|
| Mar 13 | BIZUM sent — Lavadora | -€124.03 |
| Mar 10 | BIZUM received — luz febrero | +€38.04 |
| Mar 9 | Endesa energy payment | -€38.04 |
| Mar 4 | BIZUM received — agua Febrero | +€41.34 |
| Mar 2 | Comunidad de Propietarios | -€105.92 |

- Net: **-€188.61** → `abs()` → **€188.61 shown as income** ❌
- Correct: sum of positive flows only → **€79.38** ✅

## Fix

Added a sign filter to `getCategoryBreakdown()` before the `GROUP BY`
aggregation:
- Income categories: `WHERE transactions.amount > 0`
- Expense categories: `WHERE transactions.amount < 0`

This ensures the Sankey shows the **actual gross flow** on each side
rather than a misleading abs-of-net.

## Test

Added a regression test in `CashflowAnalyticsTest` that reproduces the
exact five transactions from the reported scenario and asserts the
Sankey returns `7938` cents (€79.38) instead of the buggy `18861`
(€188.61).

app/Http/Controllers/Api/CashflowAnalyticsController.php

@@ -44,8 +44,10 @@ class CashflowAnalyticsController extends Controller
         $to = Carbon::parse($validated['to']);
         $userId = $request->user()->id;
 
-        $incomeCategories = $this->getCategoryBreakdown($userId, $from, $to, CategoryType::Income);
-        $expenseCategories = $this->getCategoryBreakdown($userId, $from, $to, CategoryType::Expense);
+        // Split by sign, not by category type: a single category can appear on
+        // both sides when it has both incoming and outgoing transactions.
+        $incomeCategories = $this->getSankeyBreakdown($userId, $from, $to, '>');
+        $expenseCategories = $this->getSankeyBreakdown($userId, $from, $to, '<');
 
         $totalIncome = $incomeCategories->sum('amount');
         $totalExpense = $expenseCategories->sum('amount');
@@ -171,12 +173,67 @@ class CashflowAnalyticsController extends Controller
             ->sum('transactions.amount');
     }
 
-    private function getCategoryBreakdown(string $userId, Carbon $from, Carbon $to, CategoryType $type)
+    private function getSankeyBreakdown(string $userId, Carbon $from, Carbon $to, string $operator)
     {
-        // Get categorized transactions
+        $isIncome = $operator === '>';
+
+        // Group all transactions by category using the amount sign, not the category
+        // type. This allows one category to appear on both sides of the Sankey when
+        // it contains transactions of both signs (e.g. an income category that also
+        // holds property expense payments will show income on the left and the
+        // outgoing payments on the right).
         $categorized = Transaction::query()
             ->where('transactions.user_id', $userId)
             ->whereBetween('transactions.transaction_date', [$from, $to])
+            ->where('transactions.amount', $operator, 0)
+            ->whereNotNull('transactions.category_id')
+            ->join('categories', 'transactions.category_id', '=', 'categories.id')
+            ->select('transactions.category_id', DB::raw('sum(transactions.amount) as total_amount'))
+            ->groupBy('transactions.category_id')
+            ->with('category')
+            ->get()
+            ->map(function ($item) {
+                return [
+                    'category_id' => $item->category_id,
+                    'category' => $item->category,
+                    'amount' => abs($item->total_amount),
+                ];
+            });
+
+        $uncategorized = Transaction::query()
+            ->where('user_id', $userId)
+            ->whereBetween('transaction_date', [$from, $to])
+            ->whereNull('category_id')
+            ->where('amount', $operator, 0)
+            ->sum('amount');
+
+        if ($uncategorized != 0) {
+            $categorized->push([
+                'category_id' => null,
+                'category' => (new Category)->forceFill([
+                    'id' => null,
+                    'name' => $isIncome ? 'Unknown Income' : 'Unknown Expense',
+                    'type' => $isIncome ? CategoryType::Income : CategoryType::Expense,
+                    'color' => 'gray',
+                    'icon' => 'HelpCircle',
+                ]),
+                'amount' => abs($uncategorized),
+            ]);
+        }
+
+        return $categorized;
+    }
+
+    private function getCategoryBreakdown(string $userId, Carbon $from, Carbon $to, CategoryType $type)
+    {
+        // Get categorized transactions — filter by sign so that outgoing payments
+        // in an income category (or refunds in an expense category) are excluded.
+        // This ensures the Sankey shows the actual gross flow for each side, not
+        // the net which could be misleading when categories contain mixed-sign entries.
+        $categorized = Transaction::query()
+            ->where('transactions.user_id', $userId)
+            ->whereBetween('transactions.transaction_date', [$from, $to])
+            ->where('transactions.amount', $type === CategoryType::Income ? '>' : '<', 0)
             ->join('categories', function ($join) use ($type) {
                 $join->on('transactions.category_id', '=', 'categories.id')
                     ->where('categories.type', '=', $type);

tests/Feature/CashflowAnalyticsTest.php

@@ -483,6 +483,85 @@ test('sankey includes unknown income and expense categories', function () {
     expect($data['expense_categories'][0]['amount'])->toBe(50000);
 });
 
+test('sankey income category with mixed positive and negative transactions shows only positive amounts', function () {
+    // Reproduces a real-world scenario where an "Income from Rents" category contains
+    // both income receipts and property-related expense payments. The Sankey should
+    // show only the actual money received (positive flows), not abs(net).
+    $incomeFromRents = Category::factory()->create([
+        'user_id' => $this->user->id,
+        'type' => CategoryType::Income,
+        'name' => 'Income from Rents',
+    ]);
+
+    $account = Account::factory()->create(['user_id' => $this->user->id]);
+
+    // Mar 13: -€124.03 — BIZUM sent (Lavadora)
+    Transaction::factory()->create([
+        'user_id' => $this->user->id,
+        'account_id' => $account->id,
+        'category_id' => $incomeFromRents->id,
+        'amount' => -12403,
+        'transaction_date' => '2026-03-13',
+    ]);
+
+    // Mar 10: +€38.04 — BIZUM received (luz febrero 2026)
+    Transaction::factory()->create([
+        'user_id' => $this->user->id,
+        'account_id' => $account->id,
+        'category_id' => $incomeFromRents->id,
+        'amount' => 3804,
+        'transaction_date' => '2026-03-10',
+    ]);
+
+    // Mar 9: -€38.04 — Endesa energy payment
+    Transaction::factory()->create([
+        'user_id' => $this->user->id,
+        'account_id' => $account->id,
+        'category_id' => $incomeFromRents->id,
+        'amount' => -3804,
+        'transaction_date' => '2026-03-09',
+    ]);
+
+    // Mar 4: +€41.34 — BIZUM received (agua Febrero 2026)
+    Transaction::factory()->create([
+        'user_id' => $this->user->id,
+        'account_id' => $account->id,
+        'category_id' => $incomeFromRents->id,
+        'amount' => 4134,
+        'transaction_date' => '2026-03-04',
+    ]);
+
+    // Mar 2: -€105.92 — Comunidad de Propietarios
+    Transaction::factory()->create([
+        'user_id' => $this->user->id,
+        'account_id' => $account->id,
+        'category_id' => $incomeFromRents->id,
+        'amount' => -10592,
+        'transaction_date' => '2026-03-02',
+    ]);
+
+    $response = $this->getJson('/api/cashflow/sankey?'.http_build_query([
+        'from' => '2026-03-01',
+        'to' => '2026-03-31',
+    ]));
+
+    $response->assertOk();
+    $data = $response->json();
+
+    // Positive flows only → income side: €38.04 + €41.34 = €79.38 (7938 cents)
+    $rentIncome = collect($data['income_categories'])->firstWhere('category.name', 'Income from Rents');
+    expect($rentIncome)->not->toBeNull();
+    expect($rentIncome['amount'])->toBe(7938);
+    expect($data['total_income'])->toBe(7938);
+
+    // Negative flows → expense side: €124.03 + €38.04 + €105.92 = €267.99 (26799 cents)
+    // The category appears on BOTH sides of the Sankey.
+    $rentExpense = collect($data['expense_categories'])->firstWhere('category.name', 'Income from Rents');
+    expect($rentExpense)->not->toBeNull();
+    expect($rentExpense['amount'])->toBe(26799);
+    expect($data['total_expense'])->toBe(26799);
+});
+
 test('breakdown includes unknown income category', function () {
     $incomeCategory = Category::factory()->create([
         'user_id' => $this->user->id,