Extrae helpers del dashboard y elimina el N+1 de saldos

La vista pasaba de 226 lineas y mezclaba tres cosas distintas: traducir los
parametros de la URL a un periodo, calcular los KPIs y montar las series de
las graficas por cuenta. Saca las dos que son autonomas a _resolve_period y
_build_account_charts y deja dashboard en 170.

De paso quita el N+1: la vista llamaba a current_balance() una vez por
cuenta para el KPI y otra vez por cuenta dentro del bucle de graficas, y
cada llamada son dos queries. Ahora los saldos salen de _account_balances,
que los calcula con dos agregados, y se reparten por un dict indexado por
id. Medido con CaptureQueriesContext: con 1/3/6 cuentas se pasa de 25/53/95
queries a 21/37/61.

accounts deja de ser un QuerySet y pasa a lista, porque ahora se recorre
mas de una vez, y la cuenta seleccionada se busca en memoria en vez de con
otra query.

Lo que sigue costando una query por cuenta es monthly_balance(), que no se
toca aqui.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

expenses_manager/expenses/views.py

@@ -112,6 +112,67 @@ def _account_balances(accounts):
     return account_balances, negative_accounts, total_balance
 
 
+def _resolve_period(request, today):
+    """Traduce los parámetros de la URL en el periodo mirado por el dashboard.
+
+    Devuelve (period, selected_year, selected_month), donde selected_month es
+    None cuando se está mirando el año completo."""
+    period = request.GET.get("period", "")
+
+    if period == "this_month":
+        return period, today.year, today.month
+
+    if period == "last_month":
+        selected_year, selected_month = sub_months(today.year, today.month, 1)
+        return period, selected_year, selected_month
+
+    if period == "this_year":
+        return period, today.year, None
+
+    selected_year = _get_int(request.GET.get("year")) or today.year
+    selected_month = _get_int(request.GET.get("month")) or None
+    return period, selected_year, selected_month
+
+
+def _build_account_charts(accounts, balance_by_account, selected_year, today):
+    """Serie mensual de saldo de cada cuenta para las gráficas del dashboard.
+
+    En el año en curso la serie se corta en el mes actual y ese último punto
+    se sustituye por el saldo real de la cuenta, que llega ya calculado en
+    balance_by_account para no repetir current_balance() por cuenta."""
+    accounts_charts = []
+
+    for acc in accounts:
+        try:
+            monthly_data = acc.monthly_balance(selected_year)
+            m_balance = [float(row["balance"]) for row in monthly_data]
+        except Exception:
+            logger.exception(
+                "Error calculando monthly_balance para la cuenta %s (año %s)",
+                acc.id, selected_year
+            )
+            m_balance = [0] * 12
+
+        current_balance = balance_by_account[acc.id]
+
+        if selected_year == today.year:
+            current_month_index = today.month - 1
+
+            if current_month_index < len(m_balance):
+                m_balance[current_month_index] = float(current_balance)
+
+            m_balance = m_balance[:today.month]
+
+        accounts_charts.append({
+            "id": acc.id,
+            "name": acc.name,
+            "data": m_balance,
+            "current_balance": current_balance,
+        })
+
+    return accounts_charts
+
+
 @login_required
 def home(request):
     today = date.today()
@@ -407,33 +468,26 @@ def expense_delete(request, pk):
 def dashboard(request):
     today = date.today()
     
-    period = request.GET.get("period", "")    
     account_id = _get_int(request.GET.get("account"))
     compare_enabled = request.GET.get("compare") == "1"
 
     # Time presets
-    if period == "this_month":
+    period, selected_year, selected_month = _resolve_period(request, today)
-        selected_year, selected_month = today.year, today.month
-    elif period == "last_month":
-        selected_year, selected_month = sub_months(today.year, today.month, 1)
-    elif period == "this_year":
-        selected_year, selected_month = today.year, None
-    else:
-        selected_year = _get_int(request.GET.get("year")) or _get_int(today.year)
-        selected_month = _get_int(request.GET.get("month"))
-        selected_month = _get_int(selected_month) if selected_month else None
 
     # Accounts
-    accounts = Account.objects.filter(owner=request.user, active=True)
+    accounts = list(Account.objects.filter(owner=request.user, active=True))
     selected_account_obj = None
     if account_id:
-        selected_account_obj = accounts.filter(id=account_id).first()
+        selected_account_obj = next((a for a in accounts if a.id == account_id), None)
 
     # Calculate the KPI
+    account_balances, _, total_balance = _account_balances(accounts)
+    balance_by_account = {row["account"].id: row["balance"] for row in account_balances}
+
     if selected_account_obj:
-        kpi_balance = selected_account_obj.current_balance()
+        kpi_balance = balance_by_account[selected_account_obj.id]
     else:
-        kpi_balance = sum(account.current_balance() for account in accounts)
+        kpi_balance = total_balance
 
     # Filter by base expenses
     expenses = Expense.objects.filter(owner=request.user)
@@ -561,32 +615,9 @@ def dashboard(request):
             })
     
     # Anual evolution by accounts
-    accounts_charts = []
+    accounts_charts = _build_account_charts(
-    for acc in accounts:
+        accounts, balance_by_account, selected_year, today
-        try:
+    )
-            monthly_data = acc.monthly_balance(selected_year)
-            m_balance = [float(row["balance"]) for row in monthly_data]
-        except Exception:
-            logger.exception(
-                "Error calculando monthly_balance para la cuenta %s (año %s)",
-                acc.id, selected_year
-            )
-            m_balance = [0] * 12
-        
-        if selected_year == today.year:
-            current_month_index = today.month - 1
-            
-            if current_month_index < len(m_balance):
-                m_balance[current_month_index] = float(acc.current_balance())
-
-            m_balance = m_balance[:today.month]
-            
-        accounts_charts.append({
-            "id": acc.id,
-            "name": acc.name,
-            "data": m_balance,
-            "current_balance": acc.current_balance(),
-        })
 
     # Goals
     goals = Goal.objects.filter(owner=request.user)