docs(research): calendar webhook support + FrankenPHP Docker setup

Resolves wayfinder tickets #2 and #3 on the calendar-backend
groundwork map (#1):

- smart-tablet-screen-calendar-webhooks.md: Nextcloud CalDAV push
  support (none usable — Mail-in-a-Box's bundled Nextcloud predates
  the Webhooks app) vs Microsoft Graph webhook subscriptions
  (supported, with renewal-job requirements).
- smart-tablet-screen-frankenphp-docker.md: FrankenPHP-based dev/prod
  Docker image and compose setup for the Symfony backend.

Also gitignore .scratch/, the repo-local scratchpad directory for
throwaway working files.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QshNimeB3TVU2vMjN7tEuz

.gitignore

@@ -60,3 +60,6 @@
 # leankg's local runtime state (pid file, index) — regenerated on start
 /.leankg/
 
+# Agent scratchpad — throwaway working files, never repo content
+/.scratch/
+