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/web/src/api.ts

@@ -227,6 +227,54 @@ export function fetchLogs(params: {
   return apiFetch(`/api/logs${qs ? `?${qs}` : ""}`);
 }
 
+// --- Backup ---------------------------------------------------------------
+// On-site encrypted DB backup. See wiki/concepts/backup-recovery.md.
+
+export interface BackupStatus {
+  configured: boolean;
+  /** Admin-chosen target directory (null = not set). */
+  targetDir: string | null;
+  /** Whether the env encryption key is present (a missing key is flagged distinctly). */
+  keyPresent: boolean;
+  running: boolean;
+  lastSuccessAt: string | null;
+  lastResult: { path: string; bytes: number; prunedFiles: number } | null;
+  lastErrorAt: string | null;
+  lastError: string | null;
+}
+
+export async function fetchBackupStatus(): Promise<BackupStatus> {
+  return apiFetch("/api/backup/status");
+}
+
+/** Set (or clear, with "") the admin-chosen target directory. Returns the new status. */
+export async function setBackupTarget(targetDir: string | null): Promise<BackupStatus> {
+  return apiFetch("/api/backup/config", { method: "PUT", body: JSON.stringify({ targetDir }) });
+}
+
+export interface TargetCheck {
+  ok: boolean;
+  /** "empty" | "missing" | "not_a_dir" | "not_writable" when !ok. */
+  reason?: string;
+}
+
+/** Probe a candidate target path server-side (exists / is a dir / is writable). */
+export async function testBackupTarget(targetDir: string): Promise<TargetCheck> {
+  return apiFetch("/api/backup/test", { method: "POST", body: JSON.stringify({ targetDir }) });
+}
+
+export interface BackupRunResult {
+  ok: true;
+  path: string;
+  bytes: number;
+  prunedFiles: number;
+}
+
+/** Trigger a manual "back up now". Throws on 409 (not configured) / 500 (run failed). */
+export async function runBackup(): Promise<BackupRunResult> {
+  return apiFetch("/api/backup/run", { method: "POST" });
+}
+
 // --- Device setup ---------------------------------------------------------
 
 export interface ConfigField {