Add PCM carrier-route postcard pick-and-send (#16)

## Summary
Closes #15.
Mirrors westfarn/web_django_template#11.

- Compose can target the postcard mailing list **or** find and select one or two USPS carrier routes by ZIP (optional radius).
- Estimated cost is shown per route and for the selection (PCM design pricing, else `PCM_POSTCARD_PRICE_*` fallback).
- Selected routes order through PCM `POST /order/postcard/with-list-count` — occupant addresses are not imported into Contacts.
- Not official EDDM postage; PCM FirstClass/Standard purchased occupant lists on carrier routes.

## Test plan
- [ ] Campaign compose: mailing-list audience still quotes list size × per-piece cost.
- [ ] Switch to USPS carrier routes, search ZIP 60540 (mocked or live PCM), pick 1–2 routes, confirm household count + estimated cost.
- [ ] Saving without a selection is rejected; saving with a quote creates one contact-less Message and shows the route panel on the report.
- [ ] Send places `/order/postcard/with-list-count` (not per-recipient `/order/postcard`).
- [ ] `uv run python manage.py test messaging.tests.CarrierRoutePostcardTests messaging.tests.PostcardDesignPickTests`

Reviewed-on: #16

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",
+            ),
+        ),
+    ]