Initial O3DE Flatpak packaging + Gitea CI

Repackage the official O3DE Linux .deb as a Flatpak and publish it as a
static OSTree repo on the 'pages' branch for install via flatpak remote.

- org.o3de.O3DE.yaml: flatpak-builder manifest (extracts the .deb into /app)
- o3de-wrapper.sh: launcher that locates the versioned o3de binary at runtime
- desktop + AppStream metadata under the app-id
- scripts/: live version resolver and local build helper
- .gitea/workflows/build-flatpak.yml: daily check -> build -> publish -> tag

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

org.o3de.O3DE.desktop

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Type=Application
+Name=O3DE
+GenericName=3D Engine
+Comment=Open 3D Engine - real-time 3D engine for games and simulations
+Exec=o3de-wrapper.sh
+Icon=org.o3de.O3DE
+Terminal=false
+Categories=Development;Graphics;3DGraphics;
+Keywords=game;engine;3d;simulation;editor;
+StartupNotify=true
+StartupWMClass=O3DE