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.

.env.example

@@ -71,6 +71,10 @@ PCM_WEBHOOK_SECRETS=
 # Quote JSON (compose/shell break on bare {…, …}).
 # PCM_RETURN_ADDRESS='{"firstName":"Jane","lastName":"Doe","address":"123 Main","city":"Town","state":"IL","zipCode":"60189"}'
 PCM_RETURN_ADDRESS=
+# Fallback $/piece when PCM design has no pricing[]; used by compose cost quotes.
+# PCM_POSTCARD_PRICE_FIRSTCLASS=0.89
+# PCM_POSTCARD_PRICE_STANDARD=0.55
+# PCM_LIST_TYPE=IRL
 
 # Stripe — required when FEATURE_PAYMENTS=true
 STRIPE_SECRET_KEY=