Add budget module and push notifications for transactions

Budget: recurring items CRUD, yearly/monthly projections with no-double-count
logic, and full UI (overview, monthly detail, recurring items manager).

Push notifications: Web Push via VAPID keys, triggered on transaction creation
from n8n. Includes service worker handlers, frontend subscription flow, and
a test button on the Dashboard (temporary).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api.ts

@@ -103,3 +103,125 @@ export interface Transaction {
   category: Category | null;
   created_at: string;
 }
+
+// --- Budget / Recurring Items ---
+
+export type RecurringItemType = 'INCOME' | 'EXPENSE' | 'SAVINGS';
+export type RecurringFrequency = 'WEEKLY' | 'MONTHLY' | 'QUARTERLY' | 'BIANNUAL' | 'YEARLY';
+
+export interface RecurringItem {
+  id: number;
+  name: string;
+  amount: number;
+  currency: string;
+  item_type: RecurringItemType;
+  frequency: RecurringFrequency;
+  day_of_month: number | null;
+  month_of_year: number | null;
+  override_amounts: Record<string, number> | null;
+  category_id: number | null;
+  is_active: boolean;
+  notes: string | null;
+  created_at: string;
+  category: Category | null;
+}
+
+export interface RecurringItemCreate {
+  name: string;
+  amount: number;
+  currency?: string;
+  item_type: RecurringItemType;
+  frequency?: RecurringFrequency;
+  day_of_month?: number | null;
+  month_of_year?: number | null;
+  override_amounts?: Record<string, number> | null;
+  category_id?: number | null;
+  is_active?: boolean;
+  notes?: string | null;
+}
+
+export interface RecurringItemUpdate {
+  name?: string;
+  amount?: number;
+  currency?: string;
+  item_type?: RecurringItemType;
+  frequency?: RecurringFrequency;
+  day_of_month?: number | null;
+  month_of_year?: number | null;
+  override_amounts?: Record<string, number> | null;
+  category_id?: number | null;
+  is_active?: boolean;
+  notes?: string | null;
+}
+
+export interface RecurringItemDetail {
+  id: number;
+  name: string;
+  amount: number;
+  projected_amount: number | null;
+  used_actual: boolean;
+  item_type: string;
+  frequency: string;
+  category_name: string | null;
+  category_id: number | null;
+}
+
+export interface ActualsBySource {
+  source: string;
+  total_compra: number;
+  total_devolucion: number;
+  net: number;
+  count: number;
+}
+
+export interface MonthlyProjection {
+  month: number;
+  year: number;
+  projected_income: number;
+  projected_fixed_expenses: number;
+  projected_savings: number;
+  actual_credit_card: number;
+  actual_cash: number;
+  actual_transfers: number;
+  uncovered_actual: number;
+  gran_total_egresos: number;
+  net_balance: number;
+}
+
+export interface YearlyProjection {
+  year: number;
+  months: MonthlyProjection[];
+  annual_income: number;
+  annual_expenses: number;
+  annual_savings: number;
+  annual_net: number;
+}
+
+export interface MonthlyDetail {
+  year: number;
+  month: number;
+  income_items: RecurringItemDetail[];
+  expense_items: RecurringItemDetail[];
+  savings_items: RecurringItemDetail[];
+  actuals_by_source: ActualsBySource[];
+  total_projected_income: number;
+  total_projected_expenses: number;
+  total_projected_savings: number;
+  uncovered_actual: number;
+  gran_total_egresos: number;
+  net_balance: number;
+}
+
+// Budget API functions
+export const getRecurringItems = (params?: { item_type?: string; is_active?: boolean }) =>
+  api.get<RecurringItem[]>('/budget/recurring', { params });
+export const createRecurringItem = (data: RecurringItemCreate) =>
+  api.post<RecurringItem>('/budget/recurring', data);
+export const updateRecurringItem = (id: number, data: RecurringItemUpdate) =>
+  api.patch<RecurringItem>(`/budget/recurring/${id}`, data);
+export const deleteRecurringItem = (id: number) =>
+  api.delete(`/budget/recurring/${id}`);
+export const getYearlyProjection = (year: number) =>
+  api.get<YearlyProjection>(`/budget/projection/${year}`);
+export const getMonthlyDetail = (year: number, month: number) =>
+  api.get<MonthlyDetail>(`/budget/month/${year}/${month}`);