feat(backup): admin-tunable retention + BACKUP_KEY as a Komodo secret

Retention (keep-last / keep-daily-days) is operational policy the on-site admin
should tune, not a server env var requiring a redeploy -- same reasoning that moved
the target directory to the UI.

- Migration 0017: site_config.backup_keep_last + backup_keep_daily_days (nullable;
  null = code default 7 / 30 per field).
- BackupService reads retention fresh each run; status() exposes keepLast +
  keepDailyDays. DEFAULT_BACKUP_RETENTION is now a pure code default (env reads gone).
- PUT /api/backup/config accepts keepLast / keepDailyDays (non-negative int, or null
  to reset to default; 400 on negative).
- UI: two retention fields on the Backup config card; one Save covers target +
  retention. i18n sq + en.

BACKUP_KEY wired into Komodo:
- komodo/resources.toml: BACKUP_KEY=[[park_buzi_backup_key]] (per-booth secret,
  alongside JWT / signing keys).
- komodo/.env.komodo.example: documents it as the ONLY backup env var -- escrow it
  offsite alongside EVENT_SIGNING_KEY (recovery needs both); target + retention are
  admin-chosen in the UI / DB, not env. Server .env.example trimmed to just BACKUP_KEY.

Also carries the small in-progress setup-intro i18n copy trim.

Tests: 218 server tests green, incl. retention persist / reset-to-default / reject-
negative and the updated status shape. Migration applies cleanly (needed a
statement-breakpoint between the two ALTERs). Wiki backup-recovery updated.

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

apps/server/.env.example

@@ -25,10 +25,9 @@ EVENT_SIGNING_KEY=
 # 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. Backups stay a no-op until BOTH this key and an in-UI
-# target directory are set.
+# target directory are set. The target directory AND retention (keep-last / keep-daily) are
+# admin-chosen in the UI (Setup → Backup), NOT env — only this key is an env secret.
 #   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