feat(accounts): home district with 6-month cooldown

- accounts/models.py: add district_changed_at DateTimeField + VALID_DISTRICTS constant (14 Kerala districts)
- migration 0013_user_district_changed_at: nullable DateTimeField, no backfill
- WebRegisterForm: accept optional district during signup, stamp district_changed_at
- UpdateProfileView: enforce 183-day cooldown with human-readable error
- LoginView/WebRegisterView/StatusView: include district_changed_at in responses

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mobile_api/forms/user_forms.py

@@ -45,7 +45,7 @@ class WebRegisterForm(forms.ModelForm):
 
     class Meta:
         model = User
-        fields = ['first_name', 'last_name', 'email', 'phone_number', 'password', 'confirm_password']
+        fields = ['first_name', 'last_name', 'email', 'phone_number', 'password', 'confirm_password', 'district']
 
     def clean_email(self):
         email = self.cleaned_data.get('email')
@@ -76,6 +76,12 @@ class WebRegisterForm(forms.ModelForm):
         # Mark as a customer / end-user
         user.is_customer = True
         user.role = 'customer'
+        from django.utils import timezone
+        from accounts.models import VALID_DISTRICTS
+        if user.district and user.district in VALID_DISTRICTS:
+            user.district_changed_at = timezone.now()
+        elif user.district:
+            user.district = None  # reject invalid district silently
         if commit:
             user.save()
         return user