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>

o3de.flatpakrepo

@@ -0,0 +1,10 @@
+[Flatpak Repo]
+Title=O3DE (unofficial Flatpak)
+# Replace <owner> with the Gitea account/org that owns this repo.
+# The CI job regenerates this file on the 'pages' branch with the correct URL,
+# so the canonical copy to use is:
+#   https://gitea.pc-heini.de/<owner>/o3de-flatpak/raw/branch/pages/o3de.flatpakrepo
+Url=https://gitea.pc-heini.de/<owner>/o3de-flatpak/raw/branch/pages
+Homepage=https://o3de.org/
+Comment=Unofficial O3DE engine repackaged as a Flatpak
+Description=Install the Open 3D Engine on any Linux distribution via Flatpak.