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/models.py

@@ -18,6 +18,7 @@ class Account(models.Model):
     name = models.CharField(max_length=100)
     account_type = models.CharField(max_length=20, choices=ACCOUNT_TYPES, default='asset')
     balance = models.DecimalField(max_digits=12, decimal_places=2, default=0.00)
+    salary_months = models.IntegerField(default=12, choices=[(12, 12), (13, 13)])
     active = models.BooleanField(default=True)
     created_at = models.DateTimeField(auto_now_add=True)
 
@@ -223,4 +224,145 @@ class BackupCode(models.Model):
         indexes = [models.Index(fields=['user', 'used'])]
 
     def __str__(self):
-        return f"{self.user} – backup {'used' if self.used else 'active'}"
+        return f"{self.user} – backup {'used' if self.used else 'active'}"
+
+
+# ── FinancialYear ─────────────────────────────────────────────────────────────
+
+class Household(models.Model):
+    name = models.CharField(max_length=100)
+    created_by = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.PROTECT,
+        related_name='created_households',
+    )
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    def __str__(self):
+        return self.name
+
+
+class HouseholdMembership(models.Model):
+    STATUS_CHOICES = [
+        ('pending', 'Pending'),
+        ('active', 'Active'),
+        ('left', 'Left'),
+    ]
+    ROLE_CHOICES = [
+        ('member', 'Member'),
+        ('admin', 'Admin'),
+    ]
+    household = models.ForeignKey(Household, on_delete=models.CASCADE, related_name='memberships')
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name='household_memberships',
+    )
+    invited_by = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.SET_NULL,
+        null=True,
+        related_name='sent_invitations',
+    )
+    status = models.CharField(max_length=10, choices=STATUS_CHOICES, default='pending')
+    role = models.CharField(max_length=10, choices=ROLE_CHOICES, default='member')
+    effective_from_year = models.PositiveSmallIntegerField(null=True, blank=True)
+    effective_until_year = models.PositiveSmallIntegerField(null=True, blank=True)
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        unique_together = ['household', 'user']
+
+    def __str__(self):
+        return f"{self.user} in {self.household} ({self.status})"
+
+
+class PendingHouseholdInvite(models.Model):
+    household = models.ForeignKey(Household, on_delete=models.CASCADE, related_name='pending_invites')
+    invited_email = models.EmailField()
+    invited_by = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.SET_NULL,
+        null=True,
+        related_name='sent_pending_invitations',
+    )
+    effective_from_year = models.PositiveSmallIntegerField(null=True, blank=True)
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        unique_together = ['household', 'invited_email']
+
+    def __str__(self):
+        return f"Pending invite for {self.invited_email} to {self.household}"
+
+
+class FinancialYear(models.Model):
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name='financial_years',
+        null=True, blank=True,
+    )
+    household = models.ForeignKey(
+        Household,
+        on_delete=models.CASCADE,
+        related_name='financial_years',
+        null=True, blank=True,
+    )
+    year = models.PositiveSmallIntegerField()
+    is_active = models.BooleanField(default=True)
+    notes = models.TextField(blank=True, default='')
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        constraints = [
+            models.CheckConstraint(
+                condition=(
+                    models.Q(user__isnull=False, household__isnull=True) |
+                    models.Q(user__isnull=True, household__isnull=False)
+                ),
+                name='financial_year_owner_exclusive',
+            ),
+            models.UniqueConstraint(
+                fields=['user', 'year'],
+                condition=models.Q(user__isnull=False),
+                name='unique_personal_financial_year',
+            ),
+            models.UniqueConstraint(
+                fields=['household', 'year'],
+                condition=models.Q(household__isnull=False),
+                name='unique_household_financial_year',
+            ),
+        ]
+
+    def __str__(self):
+        owner = self.user or self.household
+        return f"{owner} — {self.year}"
+
+
+class YearlyIncome(models.Model):
+    financial_year = models.ForeignKey(FinancialYear, on_delete=models.CASCADE, related_name='incomes')
+    member = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.SET_NULL,
+        null=True,
+        related_name='yearly_incomes',
+    )
+    name = models.CharField(max_length=100)
+    amount = models.DecimalField(max_digits=12, decimal_places=2)
+    active = models.BooleanField(default=True)
+    notes = models.TextField(blank=True, default='')
+
+    def __str__(self):
+        return f"{self.name}: CHF {self.amount} ({self.financial_year.year})"
+
+
+class YearlyBudgetItem(models.Model):
+    financial_year = models.ForeignKey(FinancialYear, on_delete=models.CASCADE, related_name='budget_items')
+    name = models.CharField(max_length=100)
+    amount = models.DecimalField(max_digits=12, decimal_places=2)
+    active = models.BooleanField(default=True)
+    notes = models.TextField(blank=True, default='')
+
+    def __str__(self):
+        return f"{self.name}: CHF {self.amount} ({self.financial_year.year})"