calendar: consume the scout-calendar feed instead of local events.json

Events now come from EVENTS_FEED_URL (the scout-calendar container,
which serves Radicale mike/site73 as JSON). The last good copy is
cached to /data/events-cache.json and served whenever the feed is down
or empty; 'Nothing on the books' remains the cold-start floor only.
The round-trip test proved the feed reproduces all 32 rows of
app/events.json byte-identically, so nothing visible changes today.

Removes app/events.json, the first-boot seed copy, and the local load
path; README calendar section rewritten; compose gains EVENTS_FEED_URL
from the Portainer stack env.

docker-compose.yml

@@ -6,8 +6,8 @@
 #   - no env_file: NTFY_URL comes from the Portainer stack env
 # Images live OUTSIDE the repo at /srv/scout-website-assets/img (bind-mounted
 # read-only) so photos never bloat clones.
-# The live, hand-editable calendar is /srv/scout-website/data/events.json
-# (seeded from app/events.json on first boot; see README).
+# The calendar comes from the scout-calendar feed (EVENTS_FEED_URL, set in
+# the Portainer stack env); /data/events-cache.json is the stale fallback.
 
 name: scout-website
 services:
@@ -25,6 +25,7 @@ services:
       - NTFY_TOPIC=${NTFY_TOPIC}
       - NTFY_TOKEN=${NTFY_TOKEN}
       - ADMIN_TOKEN=${ADMIN_TOKEN}
+      - EVENTS_FEED_URL=${EVENTS_FEED_URL}
     volumes:
       - /srv/scout-website/data:/data
       - /srv/scout-website/secrets/service_account.json:/app/service_account.json:ro