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

packages/web/vite.config.ts

@@ -13,7 +13,7 @@ export default defineConfig({
       manifest: {
         name: 'Backspace',
         short_name: 'Backspace',
-        description: 'The self-hosted Discord and TeamSpeak alternative. HD voice, video, and screen share — open-source and free.',
+        description: 'The self-hosted Discord and TeamSpeak alternative. HD voice, video, and screen share — open source and free (AGPL-3.0).',
         display: 'standalone',
         start_url: '/',
         theme_color: '#0b0b10',