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/routes/backup.routes.test.ts

@@ -49,6 +49,7 @@ describe("GET /api/backup/status", () => {
     const body = res.json();
     expect(body).toMatchObject({
       configured: false,
+      targetDir: null,
       running: false,
       lastSuccessAt: null,
       lastError: null,
@@ -56,6 +57,71 @@ describe("GET /api/backup/status", () => {
   });
 });
 
+describe("PUT /api/backup/config — admin-chosen target", () => {
+  it("403 for a user lacking backup:update", async () => {
+    const { username, password } = await seedUser(db, {
+      username: "viewer", roleId: "viewer", permissions: ["backup:read"],
+    });
+    const { cookie, csrf } = await login(app, username, password);
+    const res = await app.inject({
+      method: "PUT", url: "/api/backup/config",
+      headers: { cookie, "x-csrf-token": csrf },
+      payload: { targetDir: "/tmp/x" },
+    });
+    expect(res.statusCode).toBe(403);
+  });
+
+  it("persists the target dir and reflects it in status", async () => {
+    const { username, password } = await seedUser(db, { username: "boss", roleId: "admin" });
+    const { cookie, csrf } = await login(app, username, password);
+    const put = await app.inject({
+      method: "PUT", url: "/api/backup/config",
+      headers: { cookie, "x-csrf-token": csrf },
+      payload: { targetDir: "  /mnt/backup  " }, // trimmed server-side
+    });
+    expect(put.statusCode).toBe(200);
+    expect(put.json()).toMatchObject({ targetDir: "/mnt/backup" });
+
+    const status = await app.inject({ method: "GET", url: "/api/backup/status", headers: { cookie } });
+    expect(status.json().targetDir).toBe("/mnt/backup");
+  });
+
+  it("clears the target dir when given empty/null", async () => {
+    const { username, password } = await seedUser(db, { username: "boss", roleId: "admin" });
+    const { cookie, csrf } = await login(app, username, password);
+    await app.inject({
+      method: "PUT", url: "/api/backup/config",
+      headers: { cookie, "x-csrf-token": csrf }, payload: { targetDir: "/mnt/backup" },
+    });
+    const clear = await app.inject({
+      method: "PUT", url: "/api/backup/config",
+      headers: { cookie, "x-csrf-token": csrf }, payload: { targetDir: "" },
+    });
+    expect(clear.json().targetDir).toBeNull();
+  });
+});
+
+describe("POST /api/backup/test — path probe", () => {
+  it("reports ok for a writable directory and a reason for a missing one", async () => {
+    const { username, password } = await seedUser(db, { username: "boss", roleId: "admin" });
+    const { cookie, csrf } = await login(app, username, password);
+
+    const good = await app.inject({
+      method: "POST", url: "/api/backup/test",
+      headers: { cookie, "x-csrf-token": csrf },
+      payload: { targetDir: process.cwd() }, // an existing, writable dir
+    });
+    expect(good.json()).toMatchObject({ ok: true });
+
+    const bad = await app.inject({
+      method: "POST", url: "/api/backup/test",
+      headers: { cookie, "x-csrf-token": csrf },
+      payload: { targetDir: "/no/such/path/here-xyz" },
+    });
+    expect(bad.json()).toMatchObject({ ok: false, reason: "missing" });
+  });
+});
+
 describe("POST /api/backup/run", () => {
   it("403 for a user lacking backup:create", async () => {
     const { username, password } = await seedUser(db, {