Add PCM carrier-route postcard pick-and-send (#12)

## Summary
Closes #11.
Mirrors ai_ml_operations/monica_site#15.

- Direct-mail 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 a ZIP, 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 directmail.tests.CarrierRoutePostcardTests directmail.tests.PostcardDesignPickTests`

Reviewed-on: #12

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):