Add EDDM size, income, businesses and USPS map link (#18)

## Summary
Closes #17.

- Join USPS EDDM GIS stats onto the carrier-route picker: household size, median income, business count.
- Add Open USPS EDDM map link next to Find routes.
- GIS miss stays a dash and does not block Find routes. PCM households and cost unchanged.

Template: westfarn/web_django_template#13

## Test plan
- [ ] Find routes for a ZIP; table shows Size / Income / Businesses columns
- [ ] Missing GIS values render as —
- [ ] Open USPS EDDM map opens eddm.usps.com
- [ ] Quote and 1–2 route send still work
- [ ] `CarrierRoutePostcardTests` pass

Reviewed-on: #18

site/messaging/README.md

@@ -120,6 +120,11 @@ Route mode calls PCM `POST /list/count/zipcode` (or `/radius`) with
 or `PCM_POSTCARD_PRICE_FIRSTCLASS` / `PCM_POSTCARD_PRICE_STANDARD`. Default
 purchased list type is `IRL` (`PCM_LIST_TYPE`). This is not official EDDM postage.
 
+Compose also joins USPS EDDM GIS stats (businesses; household size / median
+income when USPS fills them) and links to
+[the EDDM map](https://eddm.usps.com/eddm/select-routes.htm) for the official
+route table.
+
 | Extra env | Purpose |
 |-----------|---------|
 | `PCM_POSTCARD_PRICE_FIRSTCLASS` | Fallback $/piece if PCM omits FirstClass pricing |