Add PCM carrier-route postcard pick-and-send (#15)

## Summary
Closes #15.
- Let compose target mailing-list contacts or 1–2 USPS carrier routes, with household count and estimated cost shown before save.
- Place selected routes through PCM list-count orders instead of importing occupant addresses into Contacts.

site/messaging/migrations/0007_carrier_route_audience.py

@@ -0,0 +1,41 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("messaging", "0006_storedfile_social_kinds"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="campaign",
+            name="route_targeting",
+            field=models.JSONField(blank=True, default=dict),
+        ),
+        migrations.AlterField(
+            model_name="campaign",
+            name="audience",
+            field=models.CharField(
+                blank=True,
+                choices=[
+                    ("email_opt_in", "Mailing list · email opt-in"),
+                    ("sms_opt_in", "Mailing list · SMS opt-in"),
+                    ("postcard_opt_in", "Mailing list · postcard opt-in"),
+                    ("carrier_route", "USPS carrier routes"),
+                ],
+                default="",
+                max_length=32,
+            ),
+        ),
+        migrations.AlterField(
+            model_name="message",
+            name="contact",
+            field=models.ForeignKey(
+                blank=True,
+                null=True,
+                on_delete=models.CASCADE,
+                related_name="messages",
+                to="contacts.contact",
+            ),
+        ),
+    ]