feat(backup): admin UI with admin-chosen target directory

The backup destination is now chosen by the on-site admin in the UI (Setup ->
Backup), not a server env var. An env-pinned target defeats the purpose: the admin
can't point backups at a freshly-plugged USB or a NAS mount without editing .env
and restarting. The encryption key stays a server secret.

Target storage:
- New site_config.backup_target_dir (migration 0016, nullable; null = not
  configured). BackupService reads it fresh each run, so a UI change takes effect
  with no restart. Only BACKUP_KEY stays env -- a key must never live in the DB it
  backs up.

Routes:
- PUT /api/backup/config  -- set/clear the target (backup:update; upserts id=1).
- POST /api/backup/test   -- probe a candidate path server-side (exists / is a
  directory / writable) so the admin gets feedback before relying on it.
- status() now exposes targetDir + keyPresent, so the UI distinguishes
  'no target set' from 'BACKUP_KEY missing'.

UI (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-key
  warning), a Back up now button, and the restore-is-out-of-band note. Full i18n
  (sq + en); nav.backup.
- API client: fetchBackupStatus / setBackupTarget / testBackupTarget / runBackup.

Also includes a small in-progress copy trim to the setup-intro i18n strings.

Verified live with Playwright: typed a path -> Test reported writable -> Save
persisted it -> status reflected it and showed the key-missing warning. Whole
monorepo build/lint/test green. Wiki backup-recovery + open-question #5 updated.

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

apps/server/src/server.ts

@@ -21,7 +21,7 @@ import { DeviceMonitor } from "./device-monitor.js";
 import { buildSigner, buildVerifier } from "./signer.js";
 import { LogService, pinoDbStream } from "./log-service.js";
 import { pruneSnapshots } from "./snapshot-retention.js";
-import { BackupService, backupConfigFromEnv } from "./backup-service.js";
+import { BackupService } from "./backup-service.js";
 import { backupRoutes } from "./routes/backup.js";
 import { logRoutes } from "./routes/logs.js";
 import { VisionClient } from "./vision-client.js";
@@ -276,11 +276,12 @@ export async function buildServer(opts: BuildOptions = {}): Promise<FastifyInsta
   // read the store (GET /api/logs, log:read). See wiki/concepts/app-logs.md.
   await logRoutes(app, logService);
 
-  // On-site encrypted DB backup (durability for the signed ledger). Admin-driven: status +
-  // a manual "back up now"; the scheduled run is the daily timer below. A no-op until
-  // BACKUP_TARGET_DIR + BACKUP_KEY are set. See wiki/concepts/backup-recovery.md.
-  const backupService = new BackupService(db, backupConfigFromEnv(), app.log);
-  await backupRoutes(app, backupService);
+  // On-site encrypted DB backup (durability for the signed ledger). Admin-driven: the target
+  // directory is admin-chosen (site_config), the key is an env secret; status + a manual "back
+  // up now"; the scheduled run is the daily timer below. A no-op until a target dir is set AND
+  // BACKUP_KEY is present. See wiki/concepts/backup-recovery.md.
+  const backupService = new BackupService(db, app.log);
+  await backupRoutes(app, db, backupService);
 
   // Periodic retention prune (age + row cap) so the log table stays bounded on the
   // offline appliance. Runs hourly; unref'd so it never holds the process open.