(config) Install native runtime requirements and split docs

Teach the systemd installer to install required Debian/Ubuntu
packages by default, with an opt-out for users who manage system
dependencies themselves.

Add explicit MariaDB install-extra handling so native installs can
pull in both the Python extra and required client build packages.

Slim down the README to production setup and operations, and move
development, Docker, migration helper, and architecture notes into
docs/development.md.

scripts/install-systemd

@@ -14,6 +14,7 @@ CSRF_TRUSTED_ORIGINS=${POBSYNC_CSRF_TRUSTED_ORIGINS:-}
 BACKUP_ROOT=${POBSYNC_BACKUP_ROOT:-/backups}
 WEB_BIND=${POBSYNC_WEB_BIND:-127.0.0.1:8010}
 FORCE_ENV=0
+INSTALL_OS_PACKAGES=1
 WITH_NGINX=0
 
 while [ "$#" -gt 0 ]; do
@@ -62,6 +63,14 @@ while [ "$#" -gt 0 ]; do
             FORCE_ENV=1
             shift
             ;;
+        --no-install-os-packages)
+            INSTALL_OS_PACKAGES=0
+            shift
+            ;;
+        --install-extras)
+            INSTALL_EXTRAS=$2
+            shift 2
+            ;;
         --with-nginx)
             WITH_NGINX=1
             shift
@@ -82,6 +91,29 @@ if [ "$(id -u)" -ne 0 ]; then
     exit 1
 fi
 
+install_os_packages() {
+    if [ "$INSTALL_OS_PACKAGES" -ne 1 ]; then
+        return
+    fi
+
+    if command -v apt-get >/dev/null 2>&1; then
+        packages="python3 python3-venv python3-pip rsync openssh-client"
+        if [ "$WITH_NGINX" -eq 1 ]; then
+            packages="$packages nginx"
+        fi
+        if [ "$INSTALL_EXTRAS" = "mariadb" ] || [ "$INSTALL_EXTRAS" = "[mariadb]" ] || [ "$INSTALL_EXTRAS" = ".[mariadb]" ]; then
+            packages="$packages default-libmysqlclient-dev build-essential pkg-config"
+        fi
+        apt-get update
+        apt-get install -y --no-install-recommends $packages
+        return
+    fi
+
+    echo "No supported package manager found; install python3, python3-venv, rsync, and openssh-client manually." >&2
+}
+
+install_os_packages
+
 if ! command -v python3 >/dev/null 2>&1; then
     echo "python3 is required." >&2
     exit 1
@@ -127,7 +159,25 @@ fi
 
 python3 -m venv "$VENV_DIR"
 "$VENV_DIR/bin/python" -m pip install --upgrade pip
-"$VENV_DIR/bin/python" -m pip install -e "$APP_DIR$INSTALL_EXTRAS"
+case "$INSTALL_EXTRAS" in
+    "")
+        pip_target=$APP_DIR
+        ;;
+    mariadb)
+        pip_target="$APP_DIR[mariadb]"
+        ;;
+    \[*\])
+        pip_target="$APP_DIR$INSTALL_EXTRAS"
+        ;;
+    .\[*\])
+        pip_target="$APP_DIR${INSTALL_EXTRAS#.}"
+        ;;
+    *)
+        echo "Unsupported install extras: $INSTALL_EXTRAS" >&2
+        exit 2
+        ;;
+esac
+"$VENV_DIR/bin/python" -m pip install -e "$pip_target"
 
 if [ ! -f "$ENV_FILE" ] || [ "$FORCE_ENV" -eq 1 ]; then
     secret=$("$VENV_DIR/bin/python" -c "import secrets; print(secrets.token_urlsafe(48))")