feat: extract deployment config into configs/ for multi-server reuse

Both borg-backup.sh and restore.sh now source a config file instead of
hardcoding paths and container names. Default is backup.conf next to the
script; override with BACKUP_CONF=/path/to/other.conf.

- configs/srv.conf     — current server (unchanged behaviour)
- configs/nexusvoice.conf — nexusvoice server (/home/acid/nexusvoice,
                            separate Scaleway path par-backup-1/nexusvoice)
- DB steps in borg-backup.sh are now guarded on [[ -n "$DB_CONTAINER" ]]
  so a config with DB_CONTAINER="" skips the dump entirely
- RUNBOOK.md §2 documents single and multi-deployment patterns

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A3rQSEidP6Y61kaCtxjVV1

restore.sh

@@ -14,25 +14,26 @@ export PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:$PATH"
 umask 077
 
 # ========================= CONFIGURATION =========================
+# All deployment-specific constants live in a config file.
+# Default: backup.conf next to this script.
+# Override: BACKUP_CONF=/path/to/other.conf restore.sh <cmd>
+# See configs/ for per-deployment examples.
 
-NAME="borg-2025"
-REPO="/home/srv/files/backups/$NAME"
-TARGET="/home/srv/files/content"
+_SCRIPT_DIR="$(cd "$(dirname "$(readlink -f "$0")")" && pwd)"
+BACKUP_CONF="${BACKUP_CONF:-${_SCRIPT_DIR}/backup.conf}"
+[[ -r "$BACKUP_CONF" ]] || { echo "ERROR: config not found: $BACKUP_CONF"; exit 1; }
+# shellcheck source=/dev/null
+source "$BACKUP_CONF"
+
+# Derived values
 ARCHIVE_TARGET_PATH="${TARGET#/}"
 
-DB_CONTAINER="mariadb"
-DB_START_TIMEOUT=180
-
-BORG_PASSPHRASE_FILE="${BORG_PASSPHRASE_FILE:-/root/.borg-passphrase}"
-ROOT_PASSWORD_FILE="${ROOT_PASSWORD_FILE:-/root/.mariadb-root.pw}"
-DUMP_SUBDIR="mariadb/dump"
-
 # Same lockfile borg-backup.sh takes (via flock -n 9) before touching $TARGET
 # or the repo, so a restore and the nightly backup cron job can never run
 # concurrently against each other.
 LOCKFILE="${LOCKFILE:-/var/lock/borg-backup.lock}"
 
-LOGDIR="${RESTORE_LOGDIR:-/var/log/borg}"
+LOGDIR="${RESTORE_LOGDIR:-${LOGDIR:-/var/log/borg}}"
 mkdir -p "$LOGDIR" 2>/dev/null || LOGDIR="/tmp"
 LOGFILE="$LOGDIR/restore-$(date +%Y-%m-%d-%H%M%S).log"