Add EDDM size, income, businesses and USPS map link (#14)

## Summary
Closes #13.

- Mirror Monica: 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.

Monica: ai_ml_operations/monica_site#17

## 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: #14

site/directmail/templates/directmail/_postcard_route_picker.html

@@ -31,14 +31,23 @@
     <input id="route-state" type="text" maxlength="2" placeholder="IL">
   </div>
   <button class="btn btn-ghost btn-sm" type="button" id="route-find-btn">Find routes</button>
+  <p class="hint" style="margin-top:8px">
+    <a id="eddm-map-link" href="https://eddm.usps.com/eddm/select-routes.htm"
+       target="_blank" rel="noopener noreferrer">Open USPS EDDM map</a>
+    — official route map. Use Show Table there for size / income filters.
+    PCM still mails a purchased occupant list, not EDDM postage.
+  </p>
   <p class="hint" id="route-status" style="margin-top:8px"></p>
-  <div id="route-table-wrap" hidden style="margin-top:12px">
+  <div id="route-table-wrap" hidden style="margin-top:12px;overflow-x:auto">
     <table class="table">
       <thead>
         <tr>
           <th>Route</th>
           <th>Area</th>
           <th>Households</th>
+          <th>Size</th>
+          <th>Income</th>
+          <th>Businesses</th>
           <th>Est. cost</th>
         </tr>
       </thead>