Add Ubuntu packaging and automated tag-based release pipeline

[sebastianelsner]

Summary

This PR adds Ubuntu/Debian packaging, desktop integration, and a complete release pipeline that publishes installable assets on vX.Y.Z tags.

Packaging added

• New Debian packaging under debian/ for all apps:
  • cameractrls (CLI + daemon + viewer + PTZ helper tools)
  • cameractrls-gtk (GTK3 launcher package)
  • cameractrls-gtk4 (GTK4 launcher package)
• Desktop integration:
  • launchers in /usr/share/applications
  • icon in /usr/share/icons/hicolor/scalable/apps
  • AppStream metainfo in /usr/share/metainfo
• Ubuntu dependency compatibility improved:
  • libturbojpeg0 | libturbojpeg

Build automation added

• Makefile target:
  • make deb builds .deb packages and places artifacts in dist/deb/

CI/CD release workflow added

• New workflow: .github/workflows/release-packages.yml
• Triggered on tags matching v*.*.*
• Steps:
  • validate tag format and tag/version consistency
  • build .deb packages
  • upload workflow artifacts
  • publish GitHub Release assets (.deb, .changes, .buildinfo)
  • add a source archive asset (cameractrls-vX.Y.Z-source.zip)
• Includes workflow_dispatch and a safety gate so actual release publishing occurs only for tags whose commit is on the default branch.

Documentation updates

• README updated with Ubuntu install instructions from GitHub Releases.
• Added PACKAGING-UBUNTU.md for local package build/install instructions.

How to make a release (maintainer flow)

1. Ensure debian/changelog top entry version matches the intended release tag (e.g. 0.6.11-1 for v0.6.11).
2. Merge release-ready changes to main.
3. Create and push a semantic tag on main:

   git tag vX.Y.Z
   git push origin vX.Y.Z

4. GitHub Actions will automatically build and publish release assets.
5. Verify release page contains:
   • cameractrls_..._all.deb
   • cameractrls-gtk_..._all.deb
   • cameractrls-gtk4_..._all.deb
   • .changes and .buildinfo
   • cameractrls-vX.Y.Z-source.zip

Validation performed

• Built packages locally with make deb and dpkg-buildpackage.
• Verified desktop/appstream metadata validation.
• Executed a full end-to-end GitHub release dry run in fork and verified published assets.