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.metainfo.xml

@@ -0,0 +1,49 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop-application">
+  <id>org.o3de.O3DE</id>
+
+  <name>O3DE</name>
+  <summary>Open 3D Engine - real-time 3D engine for games and simulations</summary>
+
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>Apache-2.0 OR MIT</project_license>
+
+  <developer id="org.o3de">
+    <name>Open 3D Foundation</name>
+  </developer>
+
+  <description>
+    <p>
+      Open 3D Engine (O3DE) is an Apache 2.0-licensed, open-source, real-time 3D
+      engine for building games and simulations, with a fully featured editor,
+      multi-threaded photorealistic renderer, and an extensible asset pipeline.
+    </p>
+    <p>
+      This is an unofficial, community-maintained Flatpak repackaging of the
+      official O3DE Linux binaries so the engine can be installed on any Linux
+      distribution, not only Debian/Ubuntu.
+    </p>
+  </description>
+
+  <launchable type="desktop-id">org.o3de.O3DE.desktop</launchable>
+
+  <url type="homepage">https://o3de.org/</url>
+  <url type="help">https://docs.o3de.org/</url>
+  <url type="bugtracker">https://github.com/o3de/o3de/issues</url>
+
+  <categories>
+    <category>Development</category>
+    <category>Graphics</category>
+  </categories>
+
+  <content_rating type="oars-1.1" />
+
+  <releases>
+    <!-- The build pipeline replaces this entry with the packaged version. -->
+    <release version="0.0.0" date="1970-01-01">
+      <description>
+        <p>Placeholder release - rewritten automatically at build time.</p>
+      </description>
+    </release>
+  </releases>
+</component>