[FR] Implement File-Based Trigger System for Automatic Mode

[yeathn]

Is your feature request related to a problem? Please describe.
Currently, Posterizarr's automatic mode can only be triggered via Docker exec command (docker exec -it posterizarr s6-setuidgid abc pwsh /config/Posterizarr.ps1). This requires direct Docker CLI access and makes automation through other containers or scripts more complex. For users running Posterizarr alongside automation tools like Autopulse, the current method is not ideal for integration.

Describe the solution you'd like
Implement a file-based trigger system for automatic mode similar to the existing recently_added watcher functionality. The system would:

Watch for specially named trigger files (e.g., auto_[timestamp].posterizarr) in the /posterizarr/watcher directory
When such a file is detected, automatically execute a full library scan
Delete the trigger file after processing

Describe alternatives you've considered
Running the Docker CLI command directly from Autopulse by adding the Docker socket as a volume (/var/run/docker.sock:/var/run/docker.sock). However, this is a security risk as it gives the Autopulse container full access to the Docker daemon, essentially providing root-equivalent access to the host system. This practice goes against container security best practices and could potentially expose the system to vulnerabilities.

Additional context
This feature would complete the automation workflow for media management:

Media is downloaded by *arr applications
Autopulse automatically adds the media to Jellyfin
Posterizarr would then automatically generate and apply custom artwork

This creates a fully automated pipeline from download to presentation, ensuring that all new media immediately gets custom artwork without manual intervention. The current workflow breaks this automation chain as there's no secure way to trigger Posterizarr's automatic mode from Autopulse, requiring either manual intervention or potentially insecure Docker socket access.