Scaffold self-hosted F-Droid repo tooling

Nix flake dev shell (fdroidserver + jdk + rsync), a NixOS module for
serving the repo via nginx/ACME, and a sync script for publishing updates.

sync.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Usage: ./sync.sh user@host:/var/lib/fdroid-repo
+# Regenerates the index, then rsyncs repo/ to the serving host.
+
+cd "$(dirname "$0")"
+
+fdroid update -c
+
+if [ $# -lt 1 ]; then
+  echo "Usage: $0 user@host:/var/lib/fdroid-repo" >&2
+  exit 1
+fi
+
+rsync -av --delete repo/ "$1/"