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

packages/db/src/schema.ts

@@ -269,6 +269,11 @@ export const siteConfig = sqliteTable("site_config", {
    *  screen; the encryption key (BACKUP_KEY) stays an env/Komodo secret and is NEVER stored
    *  here (a key must not live in the DB it backs up). See wiki/concepts/backup-recovery.md. */
   backupTargetDir: text("backup_target_dir"),
+  /** Backup retention (admin-tunable policy, not env). Keep this many newest backups always.
+   *  null ⇒ code default (7). See wiki/concepts/backup-recovery.md. */
+  backupKeepLast: integer("backup_keep_last"),
+  /** Beyond keepLast, keep one backup per day for this many days. null ⇒ code default (30). */
+  backupKeepDailyDays: integer("backup_keep_daily_days"),
   updatedAt: text("updated_at")
     .notNull()
     .default(sql`(current_timestamp)`),