(feature) Add scheduler timezone setup to the native installer

Prompt for the scheduler timezone during interactive installs and support
a --time-zone flag for scripted installs.

Detect a sensible default from POBSYNC_TIME_ZONE, timedatectl, or
/etc/timezone before falling back to UTC, and validate the value with
Python zoneinfo before writing it to pobsync.env.

Document that schedules are evaluated in POBSYNC_TIME_ZONE so production
installs can avoid UTC/local-time ambiguity.

scripts/install-systemd

@@ -17,6 +17,7 @@ if [ -n "${POBSYNC_BACKUP_ROOT:-}" ]; then
     BACKUP_ROOT_EXPLICIT=1
 fi
 WEB_BIND=${POBSYNC_WEB_BIND:-127.0.0.1:8010}
+TIME_ZONE=${POBSYNC_TIME_ZONE:-}
 FORCE_ENV=0
 INSTALL_OS_PACKAGES=1
 WITH_NGINX=0
@@ -74,6 +75,10 @@ while [ "$#" -gt 0 ]; do
             WEB_BIND=$2
             shift 2
             ;;
+        --time-zone)
+            TIME_ZONE=$2
+            shift 2
+            ;;
         --force-env)
             FORCE_ENV=1
             shift
@@ -148,6 +153,38 @@ if [ -f "$ENV_FILE" ] && [ "$FORCE_ENV" -ne 1 ] && [ "$BACKUP_ROOT_EXPLICIT" -ne
     fi
 fi
 
+detect_time_zone() {
+    if [ -n "$TIME_ZONE" ]; then
+        printf '%s\n' "$TIME_ZONE"
+        return
+    fi
+
+    if [ -n "${POBSYNC_TIME_ZONE:-}" ]; then
+        printf '%s\n' "$POBSYNC_TIME_ZONE"
+        return
+    fi
+
+    if command -v timedatectl >/dev/null 2>&1; then
+        detected=$(timedatectl show -p Timezone --value 2>/dev/null || true)
+        if [ -n "$detected" ]; then
+            printf '%s\n' "$detected"
+            return
+        fi
+    fi
+
+    if [ -f /etc/timezone ]; then
+        detected=$(sed -n '1p' /etc/timezone | tr -d '[:space:]')
+        if [ -n "$detected" ]; then
+            printf '%s\n' "$detected"
+            return
+        fi
+    fi
+
+    printf 'UTC\n'
+}
+
+TIME_ZONE=$(detect_time_zone)
+
 run_step() {
     label=$1
     shift
@@ -261,6 +298,7 @@ if [ "$INTERACTIVE" -eq 1 ]; then
     SERVICE_GROUP=$(prompt_value "Service group" "$SERVICE_GROUP")
     BACKUP_ROOT=$(prompt_value "Backup storage path" "$BACKUP_ROOT")
     WEB_BIND=$(prompt_value "Gunicorn bind address" "$WEB_BIND")
+    TIME_ZONE=$(prompt_value "Scheduler time zone" "$TIME_ZONE")
     ALLOWED_HOSTS=$(prompt_value "Allowed hosts" "$ALLOWED_HOSTS")
     CSRF_TRUSTED_ORIGINS=$(prompt_value "CSRF trusted origins, comma-separated or blank" "$CSRF_TRUSTED_ORIGINS")
     INSTALL_OS_PACKAGES=$(prompt_yes_no "Install required OS packages with apt-get" "$INSTALL_OS_PACKAGES")
@@ -331,6 +369,12 @@ if ! command -v python3 >/dev/null 2>&1; then
     exit 1
 fi
 
+if ! env POBSYNC_INSTALL_TIME_ZONE="$TIME_ZONE" python3 -c "import os; from zoneinfo import ZoneInfo; ZoneInfo(os.environ['POBSYNC_INSTALL_TIME_ZONE'])" >/dev/null 2>&1; then
+    echo "Invalid time zone: $TIME_ZONE" >&2
+    echo "Use an IANA timezone such as UTC or Europe/Amsterdam." >&2
+    exit 1
+fi
+
 if ! command -v rsync >/dev/null 2>&1; then
     echo "rsync is required." >&2
     exit 1
@@ -416,6 +460,7 @@ POBSYNC_DJANGO_CSRF_TRUSTED_ORIGINS=$CSRF_TRUSTED_ORIGINS
 
 POBSYNC_HOME=/var/lib/pobsync
 POBSYNC_BACKUP_ROOT=$BACKUP_ROOT
+POBSYNC_TIME_ZONE=$TIME_ZONE
 POBSYNC_SQLITE_PATH=/var/lib/pobsync/pobsync.sqlite3
 POBSYNC_STATIC_ROOT=/var/lib/pobsync/static