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

NOTICE

@@ -1,8 +1,9 @@
 ReplyPier
 Copyright 2024-2026 ReplyPier
 
-This product includes software developed by ReplyPier
-(https://replypier.site).
+This product is the ReplyPier AI customer-support platform: an AI agent answers
+first from your own knowledge base, a human takes over in the same conversation,
+and the visitor can rate the service at the end.
 
-Licensed under the Apache License, Version 2.0. See the LICENSE file for
-the full license text.
+Licensed under the Apache License, Version 2.0. See the LICENSE file for the
+full license text. Contribution terms are in CONTRIBUTING.md.