license: relicense to AGPL-3.0-only with commercial dual-license

- LICENSE -> verbatim GNU AGPL-3.0; add LICENSE-COMMERCIAL.md + SECURITY.md
- CLA -> exclusive-license grant (contributors keep copyright); add README
  anti-rugpull covenant + relicense record
- NOTICE / README / CONTRIBUTING / CLAUDE.md / package.json x5 updated;
  contact routed through GitHub (no email placeholders)
- AGPL section 13 source offer: operator-configurable BACKSPACE_SOURCE_URL +
  build-injected commit; sourceCodeUrl+commit on /api/instance/info;
  SourceCodeLink on login/register/settings/desktop; docs + .env.example updated

package.json

@@ -3,7 +3,7 @@
   "version": "1.0.0",
   "private": true,
   "description": "Open, self-hosted communication platform — text, voice, video, and federation",
-  "license": "Elastic-2.0",
+  "license": "AGPL-3.0-only",
   "author": "Jannis Braun",
   "homepage": "https://github.com/TheZwiss/backspace",
   "repository": {