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

wiki/concepts/backup-recovery.md

@@ -133,12 +133,15 @@ timer + the manual route**. What landed:
   **keep-last-N + one-per-day-within-N-days** (`pruneOldBackups`). Tested: round-trip decrypts to a
   **byte-identical, queryable DB**; a flipped byte or wrong key **fails GCM auth**; short key rejected;
   scratch plaintext always removed.
-- **`backup-service.ts`** — the **target directory is admin-chosen** (`site_config.backup_target_dir`,
-  migration 0016) and read **fresh each run**, so changing it in the UI takes effect with no restart.
-  Only the **encryption key stays an env/Komodo secret** (`BACKUP_KEY`) — a key must never live in the
-  DB it backs up. Retention knobs (`BACKUP_KEEP_LAST`, `BACKUP_KEEP_DAILY_DAYS`) stay env. The service
-  **serializes** concurrent runs (single in-flight guard) and records last-success / last-error;
-  `status()` exposes `targetDir` + `keyPresent` so the UI distinguishes "no target" from "no key".
+- **`backup-service.ts`** — the **target directory AND retention are admin-chosen** in the UI
+  (`site_config.backup_target_dir`, migration 0016; `backup_keep_last` + `backup_keep_daily_days`,
+  migration 0017) and read **fresh each run**, so changing them takes effect with no restart. Retention
+  columns are nullable → fall back to the code default (keep-last 7, keep-daily 30) per field. The
+  **encryption key is the ONLY backup env/Komodo secret** (`BACKUP_KEY`) — a key must never live in the
+  DB it backs up; target+retention are operational policy, not secrets. The service **serializes**
+  concurrent runs (single in-flight guard) and records last-success / last-error; `status()` exposes
+  `targetDir`, `keepLast`, `keepDailyDays` + `keyPresent` so the UI distinguishes "no target" from
+  "no key".
 - **`routes/backup.ts`** — `GET /api/backup/status` (`backup:read`); `PUT /api/backup/config` to set/
   clear the target (`backup:update`); `POST /api/backup/test` to probe a candidate path server-side —
   exists / is-a-dir / writable (`backup:update`); `POST /api/backup/run` (`backup:create`), a clean
@@ -146,9 +149,12 @@ timer + the manual route**. What landed:
   (`backup:read/update/create`) in `@parking/shared`. **No restore route** — out-of-band by design.
 - **`apps/web/src/BackupSettings.tsx`** — a Setup → **Backup** tab (gated `backup:read`): an editable
   **target-path field** with a **Test target** probe (localized ok/missing/not-a-dir/not-writable),
-  **Save**, the status panel (config state, last-run size/pruned/error, a distinct amber **missing
-  BACKUP_KEY** warning), a **Back up now** button, and the restore-is-out-of-band note. Full i18n
-  (sq + en).
+  **retention fields** (keep-last / keep-daily-days), one **Save**, the status panel (config state,
+  last-run size/pruned/error, a distinct amber **missing BACKUP_KEY** warning), a **Back up now**
+  button, and the restore-is-out-of-band note. Full i18n (sq + en).
+- **Komodo wiring.** `BACKUP_KEY` is a **per-booth Komodo secret** (`[[park_buzi_backup_key]]` in
+  `komodo/resources.toml`; documented in `komodo/.env.komodo.example`), escrowed offsite alongside
+  `EVENT_SIGNING_KEY`. It is the *only* backup env var — target + retention are in the DB.
 - **`server.ts`** — an **unref'd daily timer** (`backupService.runScheduled`), a **no-op until
   configured**, and **deliberately NOT run at startup** (a just-power-cut booth shouldn't write to a
   possibly-unmounted disk; the daily cadence + the manual button cover it).