feat: financial year planning — annual budgets, income tracking, household sharing

- Financial year page (/financial-year): year selector, 3 KPI cards (income,
  fixed costs, actual expenses), income and budget-items tabs with inline CRUD
- Revenue accounts as income source: salary-months toggle (12/13) per account
- Household support: create household, invite members by email (existing and
  new users via PendingHouseholdInvite), accept invitations, set roles
- Combined household income view across all active members
- FinancialYear, YearlyIncome, YearlyBudgetItem, Household, HouseholdMembership
  models with migrations; household invite email template
- Management command to migrate existing accounts/budgets to financial years
- FinancialYearService in Angular with full API integration
- Dashboard updated: income/fixed-costs read from financial year data,
  year dropdown synced with available financial years
- Sidebar: financial year nav item added
- i18n: all keys in DE/EN/FR/IT

backend/finance/migrations/0020_household_membership_role.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.4 on 2026-05-19 07:24
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('finance', '0019_financial_year'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='householdmembership',
+            name='role',
+            field=models.CharField(choices=[('member', 'Member'), ('admin', 'Admin')], default='member', max_length=10),
+        ),
+    ]