feat: add durable chat storage and disconnect recovery

scripts/install_postgresql_storage.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+usage() {
+  printf '%s\n' 'Usage: install_postgresql_storage.sh --check|--install'
+}
+
+case "${1:-}" in
+  --check)
+    if ! command -v psql >/dev/null 2>&1; then
+      printf '%s\n' 'PostgreSQL is not installed.'
+      exit 1
+    fi
+    psql --version
+    if command -v pg_lsclusters >/dev/null 2>&1; then
+      pg_lsclusters
+    fi
+    ;;
+  --install)
+    if [ "$(id -u)" -ne 0 ]; then
+      printf '%s\n' 'PostgreSQL installation requires root.' >&2
+      exit 1
+    fi
+    export DEBIAN_FRONTEND=noninteractive
+    codename=''
+    if [ -r /etc/os-release ]; then
+      . /etc/os-release
+      codename="${VERSION_CODENAME:-}"
+    fi
+    if [ -z "$codename" ]; then
+      printf '%s\n' 'Could not determine the Ubuntu/Debian codename.' >&2
+      exit 1
+    fi
+    if [ -x /usr/share/postgresql-common/pgdg/apt.postgresql.org.sh ]; then
+      bash /usr/share/postgresql-common/pgdg/apt.postgresql.org.sh -y "$codename"
+    fi
+    apt-get update
+    candidate="$(apt-cache policy postgresql-18 2>/dev/null || true)"
+    case "$candidate" in
+      *'Candidate: (none)'*|"") apt-get install -y postgresql postgresql-client ;;
+      *) apt-get install -y postgresql-18 postgresql-client-18 ;;
+    esac
+    systemctl enable --now postgresql
+    printf '%s\n' 'Native PostgreSQL installation completed.'
+    psql --version
+    ;;
+  *)
+    usage >&2
+    exit 2
+    ;;
+esac