feat: insurance section — overview, documents, analysis, KVG premium comparison

- Insurance overview page (/insurance): current policies table with type,
  provider, premium, franchise, coverage, and document links
- Documents page: upload and manage insurance documents
- Analysis page: coverage gap analysis per insurance type
- Priminfo integration (/insurance/priminfo): KVG premium comparison by
  insurer, model (TAR/HMO/etc.), franchise level, and accident coverage
  via embedded Priminfo iframe (no public API available)
- Backend: Insurance, PraemienEntry, PraemienPolice models with migrations
- Sidebar: insurance nav group with flyout and dropdown
- i18n: all keys in DE/EN/FR/IT

backend/finance/serializers.py

@@ -1,6 +1,6 @@
 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, Insurance
 
 User = get_user_model()
 
@@ -56,6 +56,12 @@ class DeadlineSerializer(serializers.ModelSerializer):
         exclude = ['user']
 
 
+class InsuranceSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = Insurance
+        exclude = ['user']
+
+
 class RegisterSerializer(serializers.Serializer):
     email = serializers.EmailField()
     password = serializers.CharField(min_length=8, write_only=True)