refactor: rename Fabled Assistant to Fabled Scribe

Updates user-visible branding across frontend (PWA manifest, page title,
Settings, push fallback), backend (email templates and subjects, LLM
system prompt, CalDAV displayname, SMTP from-name default), README,
quickstart compose, and MCP server description.

Also updates the CI image path and quickstart image reference to
git.fabledsword.com/bvandeusen/fabledscribe in preparation for the
Forgejo repo rename. Internal Python package, env vars, and database
schema unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/public/manifest.json

@@ -1,6 +1,6 @@
 {
-  "name": "Fabled Assistant",
-  "short_name": "Fabled",
+  "name": "Fabled Scribe",
+  "short_name": "Scribe",
   "description": "Your self-hosted second brain with AI assistance",
   "start_url": "/",
   "display": "standalone",