Add PCM carrier-route postcard pick-and-send (#11)

## Summary
Closes #11.
- Mirror Monica: compose can use the postcard mailing list or 1–2 USPS carrier routes, with estimated cost shown.
- Place selected routes through PCM list-count orders instead of importing occupant addresses.

site/directmail/models.py

@@ -27,6 +27,7 @@ class Campaign(UUIDPrimaryKeyModel, TimeStampedModel):
 
     class Audience(models.TextChoices):
         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)
@@ -58,6 +59,8 @@ class Campaign(UUIDPrimaryKeyModel, TimeStampedModel):
     body_override = models.TextField(blank=True)
     # Set when completion 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"]
@@ -83,7 +86,11 @@ class Message(UUIDPrimaryKeyModel, TimeStampedModel):
         Campaign, on_delete=models.CASCADE, related_name="messages"
     )
     contact = models.ForeignKey(
-        Contact, on_delete=models.CASCADE, related_name="directmail_messages"
+        Contact,
+        null=True,
+        blank=True,
+        on_delete=models.CASCADE,
+        related_name="directmail_messages",
     )
     channel = models.CharField(max_length=16, choices=Channel.choices)
     status = models.CharField(
@@ -100,7 +107,8 @@ 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):