feat(#49): add geolocation (latitude/longitude) to municipalities

- Municipality model with optional latitude/longitude decimal fields
- Municipality and brief serializers expose lat/lng (also in product_provenance of public/authenticated summaries for the Leaflet map)
- seed_geography endpoint reads lat/lng and updates existing municipalities on reseed (idempotent)
- TDD tests for model, serializers, summaries and seed

tienda_ilusion/don_confiao/models/geography.py

@@ -39,6 +39,12 @@ class Municipality(models.Model):
     country = models.ForeignKey(
         Country, on_delete=models.PROTECT, related_name="municipalities"
     )
+    latitude = models.DecimalField(
+        max_digits=10, decimal_places=7, null=True, blank=True
+    )
+    longitude = models.DecimalField(
+        max_digits=10, decimal_places=7, null=True, blank=True
+    )
 
     class Meta:
         verbose_name = "Municipality"