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/templates/emails/household_invite.txt

@@ -0,0 +1,10 @@
+Hallo {{ invitee_name }},
+
+{{ inviter_name }} hat dich eingeladen, dem Haushalt "{{ household_name }}" auf Armarium beizutreten.
+
+{{ cta_label }}:
+{{ accept_url }}
+
+Falls du diese Einladung nicht erwartet hast, kannst du sie ignorieren.
+
+– Das Armarium-Team