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/0022_add_pending_household_invite.py

@@ -0,0 +1,30 @@
+# Generated by Django 6.0.4 on 2026-05-21 19:49
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('finance', '0021_add_salary_months_to_account'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='PendingHouseholdInvite',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('invited_email', models.EmailField(max_length=254)),
+                ('effective_from_year', models.PositiveSmallIntegerField(blank=True, null=True)),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('household', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='pending_invites', to='finance.household')),
+                ('invited_by', models.ForeignKey(null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='sent_pending_invitations', to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'unique_together': {('household', 'invited_email')},
+            },
+        ),
+    ]