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/serializers.py

@@ -1,6 +1,10 @@
 from rest_framework import serializers
 from django.contrib.auth import get_user_model
-from .models import Account, Transaction, Budget, Expense, Profile, Deadline
+from .models import (
+    Account, Transaction, Budget, Expense, Profile, Deadline,
+    Household, HouseholdMembership, PendingHouseholdInvite,
+    FinancialYear, YearlyIncome, YearlyBudgetItem,
+)
 
 User = get_user_model()
 
@@ -44,6 +48,10 @@ class ExpenseSerializer(serializers.ModelSerializer):
 
 class ProfileSerializer(serializers.ModelSerializer):
     totp_enabled = serializers.BooleanField(read_only=True)
+    email = serializers.SerializerMethodField()
+
+    def get_email(self, obj):
+        return obj.email or (obj.user.email if obj.user else '')
 
     class Meta:
         model = Profile
@@ -56,6 +64,76 @@ class DeadlineSerializer(serializers.ModelSerializer):
         exclude = ['user']
 
 
+class HouseholdMembershipSerializer(serializers.ModelSerializer):
+    user_email = serializers.EmailField(source='user.email', read_only=True)
+    invited_by_email = serializers.EmailField(source='invited_by.email', read_only=True)
+
+    class Meta:
+        model = HouseholdMembership
+        fields = ['id', 'user', 'user_email', 'invited_by_email', 'status', 'role',
+                  'effective_from_year', 'effective_until_year', 'created_at']
+        read_only_fields = ['id', 'user', 'user_email', 'invited_by_email', 'created_at']
+
+
+class PendingHouseholdInviteSerializer(serializers.ModelSerializer):
+    invited_by_email = serializers.EmailField(source='invited_by.email', read_only=True)
+
+    class Meta:
+        model = PendingHouseholdInvite
+        fields = ['id', 'invited_email', 'invited_by_email', 'effective_from_year', 'created_at']
+        read_only_fields = fields
+
+
+class HouseholdSerializer(serializers.ModelSerializer):
+    memberships = HouseholdMembershipSerializer(many=True, read_only=True)
+    pending_invites = PendingHouseholdInviteSerializer(many=True, read_only=True)
+    created_by_email = serializers.EmailField(source='created_by.email', read_only=True)
+
+    class Meta:
+        model = Household
+        fields = ['id', 'name', 'created_by_email', 'memberships', 'pending_invites', 'created_at']
+        read_only_fields = ['id', 'created_by_email', 'memberships', 'pending_invites', 'created_at']
+
+
+class YearlyIncomeSerializer(serializers.ModelSerializer):
+    member_email = serializers.EmailField(source='member.email', read_only=True)
+
+    class Meta:
+        model = YearlyIncome
+        fields = ['id', 'member', 'member_email', 'name', 'amount', 'active', 'notes']
+        read_only_fields = ['id', 'member_email']
+
+
+class YearlyBudgetItemSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = YearlyBudgetItem
+        fields = ['id', 'name', 'amount', 'active', 'notes']
+        read_only_fields = ['id']
+
+
+class FinancialYearSerializer(serializers.ModelSerializer):
+    incomes = YearlyIncomeSerializer(many=True, read_only=True)
+    budget_items = YearlyBudgetItemSerializer(many=True, read_only=True)
+    total_income = serializers.SerializerMethodField()
+    total_fixed_costs = serializers.SerializerMethodField()
+    owner_type = serializers.SerializerMethodField()
+
+    class Meta:
+        model = FinancialYear
+        fields = ['id', 'year', 'is_active', 'notes', 'owner_type', 'household_id',
+                  'total_income', 'total_fixed_costs', 'incomes', 'budget_items', 'created_at']
+        read_only_fields = ['id', 'created_at', 'owner_type', 'household_id', 'total_income', 'total_fixed_costs']
+
+    def get_total_income(self, obj):
+        return sum(i.amount for i in obj.incomes.filter(active=True))
+
+    def get_total_fixed_costs(self, obj):
+        return sum(b.amount for b in obj.budget_items.filter(active=True))
+
+    def get_owner_type(self, obj):
+        return 'household' if obj.household_id else 'personal'
+
+
 class RegisterSerializer(serializers.Serializer):
     email = serializers.EmailField()
     password = serializers.CharField(min_length=8, write_only=True)
@@ -67,8 +145,21 @@ class RegisterSerializer(serializers.Serializer):
 
     def create(self, validated_data):
         email = validated_data['email']
-        return User.objects.create_user(
+        user = User.objects.create_user(
             username=email,
             email=email,
             password=validated_data['password'],
         )
+        from .models import PendingHouseholdInvite, HouseholdMembership
+        for invite in PendingHouseholdInvite.objects.filter(invited_email__iexact=email):
+            HouseholdMembership.objects.get_or_create(
+                household=invite.household,
+                user=user,
+                defaults={
+                    'invited_by': invite.invited_by,
+                    'status': 'pending',
+                    'effective_from_year': invite.effective_from_year,
+                },
+            )
+            invite.delete()
+        return user