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>

accounts/migrations/0013_user_district_changed_at.py

@@ -0,0 +1,16 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('accounts', '0012_user_eventify_id'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='user',
+            name='district_changed_at',
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+    ]