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

RUNBOOK.md

@@ -50,21 +50,43 @@ Run once, by hand, on the server:
 
 ## 2. Deploying the Scripts
 
-Copy `borg-backup.sh`, `dump_db.sh`, and `restore.sh` to `/opt/backup-agent/`
-(this exact path is what `borg-backup.sh` invokes for `dump_db.sh`). Make
-all three executable:
+### Single deployment (default)
+
+Copy `borg-backup.sh`, `dump_db.sh`, `restore.sh`, and your chosen config
+from `configs/` to `/opt/backup-agent/`. Symlink the config as `backup.conf`
+next to the scripts, or set `BACKUP_CONF` in the cron entry:
 
 ```bash
 chmod +x /opt/backup-agent/borg-backup.sh /opt/backup-agent/restore.sh
 chmod +x /opt/backup-agent/dump_db.sh
+# Option A — symlink (scripts auto-discover backup.conf beside them):
+ln -s /opt/backup-agent/configs/srv.conf /opt/backup-agent/backup.conf
+# Option B — explicit env var in the cron entry (see §3).
 ```
 
+### Multiple deployments on different servers
+
+Each server gets its own config file. Deploy the same three scripts to
+`/opt/backup-agent/` on each server. Point each server's cron entry at its
+config via `BACKUP_CONF`:
+
+```
+# /etc/cron.d/borg-backup  (nexusvoice server)
+BACKUP_CONF=/opt/backup-agent/configs/nexusvoice.conf
+30 2 * * * root /opt/backup-agent/borg-backup.sh >> /var/log/borg/cron.log 2>&1
+```
+
+Before running, verify/update `configs/nexusvoice.conf`:
+- `TARGET` — confirm `/home/acid/nexusvoice` (or `/opt/nexusvoice` after the move)
+- `DB_CONTAINER` — confirm the MariaDB container name on that server
+- `REPO` — must not overlap with the srv repo; uses separate Scaleway path
+
 ## 3. Scheduling
 
 Add a cron entry to run the backup daily, off-peak:
 
 ```
-# /etc/cron.d/borg-backup
+# /etc/cron.d/borg-backup  (srv — default config via symlink)
 30 2 * * * root /opt/backup-agent/borg-backup.sh >> /var/log/borg/cron.log 2>&1
 ```