Block example.com emails and homesites under 10

Spam submissions were still getting through with placeholder data. Two
more content filters on the public form endpoints:

- Reject emails from reserved documentation domains (example.com/.org/
  /.net/.edu and subdomains) and reserved TLDs (.test/.example/.invalid/
  localhost). testing@example.com and friends are never real leads.
- Reject a homesites count below 10. Real associations are larger; the
  junk uses 0/1/2.

Both are validated server-side in security.js (validateEmail gains a
domain blocklist, new validateHomesites) and mirrored client-side in
app.js for immediate feedback. The homesites input min attribute goes
from 1 to 10. Blocked submissions return 400 with a `field` hint and
store nothing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

index.html

@@ -469,7 +469,7 @@
         <div class="calc-grid">
           <div class="calc-field">
             <label for="calcHomesites">Number of homesites</label>
-            <input type="number" id="calcHomesites" placeholder="e.g. 150" min="1" />
+            <input type="number" id="calcHomesites" placeholder="e.g. 150" min="10" />
           </div>
           <div class="calc-field">
             <label for="calcPropertyType">Property type</label>