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-wrapper.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+# Launcher for the O3DE Project Manager inside the Flatpak sandbox.
+#
+# The Debian package installs O3DE under /opt/O3DE/<version>/, which becomes
+# /app/opt/O3DE/<version>/ inside the Flatpak. The version directory name
+# changes with every release, so we discover the executable at runtime instead
+# of hard-coding a path.
+set -eu
+
+O3DE_ROOT=/app/opt/O3DE
+
+# Bundled shared libraries that ship inside the .deb.
+export LD_LIBRARY_PATH="/app/lib:${O3DE_ROOT}/lib:${LD_LIBRARY_PATH:-}"
+
+# Locate the Project Manager executable ("o3de").
+O3DE_BIN=$(find "$O3DE_ROOT" -type f -name o3de -path '*bin/Linux*' 2>/dev/null | head -n 1)
+if [ -z "$O3DE_BIN" ]; then
+    O3DE_BIN=$(find "$O3DE_ROOT" -type f -executable -name o3de 2>/dev/null | head -n 1)
+fi
+
+if [ -z "$O3DE_BIN" ]; then
+    echo "error: O3DE executable not found under $O3DE_ROOT" >&2
+    echo "       (the .deb layout may have changed)" >&2
+    exit 1
+fi
+
+# Make libraries that sit next to the binary discoverable too.
+export LD_LIBRARY_PATH="$(dirname "$O3DE_BIN"):${LD_LIBRARY_PATH}"
+
+exec "$O3DE_BIN" "$@"