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/tasks.py

@@ -18,7 +18,9 @@ def send_campaign_message(message_id: str) -> None:
 
     campaign = message.campaign
 
-    if not contact_may_receive(message.contact, message.channel):
+    if message.contact_id is None:
+        pass
+    elif not contact_may_receive(message.contact, message.channel):
         message.status = Message.Status.SUPPRESSED
         message.error = "Contact opted out or suppressed"
         message.save(update_fields=["status", "error", "updated_at"])