feat(backup): encrypted on-site DB backup engine + local target

The SQLite DB is the signed append-only ledger, so a disk failure / stolen or
destroyed PC means total revenue-history loss (open-question #5). This is the first
slice of the backup-recovery design: the engine + a local/mounted target + a daily
timer + a manual route.

Engine (apps/server/src/backup.ts):
- Consistent online copy of the live WAL DB via better-sqlite3's native .backup()
  (not a raw file copy, which can capture a torn WAL) — the restored copy is a
  byte-identical, queryable DB.
- AES-256-GCM with a scrypt-derived key from BACKUP_KEY; self-describing header
  (magic|version|salt|iv|...|authTag) so a restore tool needs only the key + file.
  Zero new dependencies (Node crypto).
- The plaintext intermediate is kept in scratch (not the removable/network target)
  and wiped in a finally, success or fail.
- Retention: keep-last-N + one-per-day within N days.

Wiring:
- BackupService (env config, single in-flight guard, last-success/last-error).
- routes/backup.ts: GET /api/backup/status (backup:read), POST /api/backup/run
  (backup:create), 409 when unconfigured. No restore route — restore is an
  out-of-band runbook action on a fresh appliance, not a console call.
- New  permission resource in @parking/shared.
- server.ts: an unref'd daily timer, a no-op until BACKUP_TARGET_DIR + BACKUP_KEY
  are set, deliberately not run at startup (a just-power-cut booth shouldn't write
  to a possibly-unmounted disk).
- openRawDb() added to @parking/db/testing (open a file without migrating, for
  restore-verification tests).

BACKUP_KEY is deliberately SEPARATE from EVENT_SIGNING_KEY (independent rotation;
backups travel, the signing key shouldn't). SMB/NFS work as mount paths; SFTP +
admin UI + restore runbook are deferred slices. Tests: round-trip byte-identical,
GCM tamper/wrong-key fail, short-key rejected, scratch cleaned, route auth/RBAC +
409. build/lint/test green (212 server tests). Wiki + open-question #5 updated.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/server/.env.example

@@ -15,6 +15,19 @@ JWT_SECRET=
 # them (keyId), so verifyChain still validates a chain that spans a key change.
 EVENT_SIGNING_KEY=
 
+# On-site encrypted DB backup (durability for the signed ledger). Disabled until both
+# BACKUP_TARGET_DIR and BACKUP_KEY are set. A daily timer + an admin "back up now" button
+# write a consistent, AES-256-GCM-encrypted copy to the target. RESTORE is an out-of-band
+# runbook action, not a console call. See wiki/concepts/backup-recovery.md.
+#   BACKUP_TARGET_DIR=/mnt/backup     # a mounted local/USB/SATA/SMB/NFS path
+# Dedicated backup-encryption key (>=16 chars), SEPARATE from EVENT_SIGNING_KEY so it can
+# rotate without fracturing the signed chain. Generate with:  openssl rand -hex 32
+# Escrow it offsite (alongside EVENT_SIGNING_KEY) — recovery needs both, and neither is ever
+# stored inside the backup it unlocks.
+#   BACKUP_KEY=
+#   BACKUP_KEEP_LAST=7                # keep this many newest backups always
+#   BACKUP_KEEP_DAILY_DAYS=30         # plus one-per-day within this window
+
 # Optional ----------------------------------------------------------------
 # PORT=3000
 # HOST=0.0.0.0                 # interface to bind. 127.0.0.1 = loopback only.