(ops) Install pobsync-manage for native management commands

Add a pobsync-manage wrapper that loads the native environment file before
running Django management commands, so production commands use the same
database and runtime settings as the systemd services.

Install the wrapper from the systemd installer, use it for migrations,
static collection, SSH key setup, and superuser creation, and document it
in the README for operational commands.

deploy/bin/pobsync-manage

@@ -0,0 +1,19 @@
+#!/bin/sh
+set -eu
+
+APP_DIR="@POBSYNC_APP_DIR@"
+VENV_DIR="@POBSYNC_VENV_DIR@"
+ENV_FILE="@POBSYNC_ENV_FILE@"
+
+if [ ! -f "$ENV_FILE" ]; then
+    echo "pobsync environment file not found: $ENV_FILE" >&2
+    exit 1
+fi
+
+set -a
+# shellcheck disable=SC1090
+. "$ENV_FILE"
+set +a
+
+cd "$APP_DIR"
+exec "$VENV_DIR/bin/python" "$APP_DIR/manage.py" "$@"