[tibroc]

virtUOS modifications rebased onto upstream v4.20.0 (THM-Health/PILOS 4.x). This PR documents our state vs upstream and triggers the container image build; it is not meant to be merged.

Fork changes:

• run only docker jobs: removes upstream workflows/dependabot; only virtuos-docker.yml builds and pushes the image
• All guests can be moderators: guests get the room's default_role; lobby restricted to enabled/disabled ("only for guests" option removed from room settings, room type admin and useRoomTypeSettings.js)
• Allow logins without first name attribute: missing first_name gets a placeholder instead of failing the login

Upstream changes relevant for deployment (4.19.0 / 4.20.0):

• APP_URL must be the exact public URL; requests for other hostnames are rejected
• Breaking meeting end callback signature: running meetings during the upgrade end via server poll (~1 min)
• New migrations: room file names are sanitized, existing password reset links are invalidated
• nginx in the image only executes index.php and forces downloads/strict CSP for uploaded files