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/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):