ci: a push here should trigger a backend sync_db and a frontend rebuild

[avivace]

Every push to the tracked databases (currently this repo and the GBA over https://github.com/gbadev-org/games) should trigger:

• A git pull of database that received a push on the deployment
• A runscript sync_db on the Django container on the deployment
• A rebuild of the frontend. This should be feasible triggering the workflow_dispatch event over https://github.com/gbdev/virens

As such CI/CD setup this involves events and GH Actions workflows in 3 different repositories, this must be documented (ideally in the main repository, https://github.com/gbdev/homebrewhub, starting from the general idea and explaining our instance of setup/tracked database repositories + SSH keys set-up)