chore: settle product naming across app, widget and deployments

One word everywhere: ReplyPier in UI copy, package and image names, the
replypier.site domains (app/api/docs/help), REPLYPIER_* settings and the
--rp-* design tokens.

- public widget API is window.ReplyPier with replypierId / replypierBaseUrl /
  replypierConfig / replypierToken, loader at webclient/replypier.min.js;
  both bundles rebuilt from source
- NOTICE, llms.txt, CONTRIBUTING, HELP_CENTER_INFRA and the demo runtime
  config rewritten; the demo config ships no third-party keys
- migration adds the ReplyPier managed model value to the AI model enum
- local compose project, database, role, volume and Docker network carry the
  same name

backend/tests/services/test_domain_verification.py

@@ -151,7 +151,7 @@ def test_domain_endpoints_flow(client, db, test_organization):
     assert body["custom_domain"] == "help.customer.com"
     assert body["domain_status"] == "pending"
     assert [r["type"] for r in body["records"]] == ["CNAME", "TXT"]
-    assert body["records"][1]["host"] == "_replypier.customer.com"
+    assert body["records"][1]["host"] == "_replypier.help.customer.com"
 
     with patch.object(dv, "check_txt_record", return_value=True), \
          patch.object(dv, "check_cname_record", return_value=True), \