chore: update Docker and CI for web + backend architecture

Replace scraper container with backend API container. Web image no
longer mounts a data volume or ships SQLite. Backend image runs Hono
server with node-cron scheduler, owns the database exclusively.

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

.dockerignore

@@ -5,6 +5,7 @@ node_modules
 data/*.db
 data/*.db-shm
 data/*.db-wal
+backend/data
 .env*
 npm-debug.log*
 .DS_Store