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/models.py

@@ -29,6 +29,7 @@ class Campaign(UUIDPrimaryKeyModel, TimeStampedModel):
         EMAIL_OPT_IN = "email_opt_in", "Mailing list · email opt-in"
         SMS_OPT_IN = "sms_opt_in", "Mailing list · SMS opt-in"
         POSTCARD_OPT_IN = "postcard_opt_in", "Mailing list · postcard opt-in"
+        CARRIER_ROUTE = "carrier_route", "USPS carrier routes"
 
     name = models.CharField(max_length=120)
     channel = models.CharField(max_length=16, choices=Channel.choices)
@@ -59,6 +60,8 @@ class Campaign(UUIDPrimaryKeyModel, TimeStampedModel):
     body_override = models.TextField(blank=True)
     # Set when realtor summary email is sent (campaign COMPLETED).
     notify_sent_at = models.DateTimeField(null=True, blank=True)
+    # PCM purchased-list targeting (ZIP/CRRT codes, listCountID, quote).
+    route_targeting = models.JSONField(default=dict, blank=True)
 
     class Meta:
         ordering = ["-created_at"]
@@ -84,7 +87,7 @@ class Message(UUIDPrimaryKeyModel, TimeStampedModel):
         Campaign, on_delete=models.CASCADE, related_name="messages"
     )
     contact = models.ForeignKey(
-        Contact, on_delete=models.CASCADE, related_name="messages"
+        Contact, null=True, blank=True, on_delete=models.CASCADE, related_name="messages"
     )
     channel = models.CharField(max_length=16, choices=Channel.choices)
     status = models.CharField(
@@ -101,7 +104,8 @@ class Message(UUIDPrimaryKeyModel, TimeStampedModel):
         ordering = ["-created_at"]
 
     def __str__(self) -> str:
-        return f"{self.channel} → {self.contact} ({self.status})"
+        who = self.contact or "carrier-route mailing"
+        return f"{self.channel} → {who} ({self.status})"
 
 
 class ProviderEvent(TimeStampedModel):