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>

.gitignore

@@ -0,0 +1,14 @@
+# flatpak-builder build artifacts
+build-dir/
+.flatpak-builder/
+repo/
+
+# downloaded payload
+*.deb
+*.deb.sha256
+*.flatpak
+o3de.deb
+
+# misc
+.DS_Store
+*.log