Fix auth persistence: web client session timeout + bot cache loss

Web client: trustDevice now defaults to true so a refresh token is always
issued on login, preventing deauth after the 1-hour access token expiry.
Users can still uncheck the box on shared devices.

Bot: cache file path is now env-configurable (BOT_CACHE_FILE) and
defaults to /app/cache/user_cache.pkl. Docker Compose mounts a named
volume at /app/cache so the session cache survives container restarts.
saveCache() now creates the directory if it doesn't exist.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -42,6 +42,8 @@ services:
     depends_on:
       app:
         condition: service_started
+    volumes:
+      - botcache:/app/cache
 
   client:
     build:
@@ -56,3 +58,4 @@ services:
 
 volumes:
   pgdata:
+  botcache: