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/README.md

@@ -110,6 +110,22 @@ HMAC-SHA256 of body) matching **any** listed secret.
 Correlation: we send `extRefNbr=<Message.uuid>` on each recipient; webhooks should
 echo that (or `orderID`, matched to `Message.provider_message_id`).
 
+### Carrier-route (neighborhood) mailings
+
+Postcard compose can target **mailing-list contacts** or **USPS carrier routes**.
+Route mode calls PCM `POST /list/count/zipcode` (or `/radius`) with
+`breakdownType=ZipCRRT`, then recounts selected routes via
+`POST /list/count/carrier-route` and sends
+`POST /order/postcard/with-list-count`. Estimated cost uses design `pricing[]`
+or `PCM_POSTCARD_PRICE_FIRSTCLASS` / `PCM_POSTCARD_PRICE_STANDARD`. Default
+purchased list type is `IRL` (`PCM_LIST_TYPE`). This is not official EDDM postage.
+
+| Extra env | Purpose |
+|-----------|---------|
+| `PCM_POSTCARD_PRICE_FIRSTCLASS` | Fallback $/piece if PCM omits FirstClass pricing |
+| `PCM_POSTCARD_PRICE_STANDARD` | Fallback $/piece for Standard |
+| `PCM_LIST_TYPE` | PCM list type key (default `IRL`) |
+
 ### Campaign completion email
 
 When a campaign reaches **completed** (email, SMS, or postcard), one summary email