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

komodo/resources.toml

@@ -15,7 +15,8 @@
 #
 # Secrets ([[park_buzi_jwt_secret]] etc.) are REFERENCES to Komodo Core's secret store —
 # per-booth + unique, never inlined here (this file is in git). JWT_SECRET gates login;
-# EVENT_SIGNING_KEY signs the append-only anti-fraud ledger.
+# EVENT_SIGNING_KEY signs the append-only anti-fraud ledger; BACKUP_KEY encrypts on-site DB
+# backups (separate from the signing key; escrow it offsite — recovery needs both).
 #
 # Deploys are MANUAL + PINNED in spirit: bump TAG to an immutable dev-<sha> before a
 # production booth goes live (TAG=dev here is the moving tag, fine while staging). NO
@@ -48,4 +49,5 @@ VISION_ENABLED=1
 WS_ALLOWED_ORIGINS=
 JWT_SECRET=[[park_buzi_jwt_secret]]
 EVENT_SIGNING_KEY=[[park_buzi_event_signing_key]]
+BACKUP_KEY=[[park_buzi_backup_key]]
 """