feat(recovery): add safe-mode recovery surface and emergency CLI (#1286)

* feat(recovery): add safe-mode recovery surface and emergency CLI

Add a read-only Recovery tab under Settings (admin-only) backed by a new
GET /api/diagnostics endpoint reporting app version, database integrity,
encryption-key status, Docker reachability, account and SSO counts, and
non-secret configuration. The endpoint loads without Docker or live metrics
so it stays available when the dashboard does not, requires a genuine admin
session, and builds its config block from a non-secret allowlist so no
credentials are ever exposed.

Expand the emergency command-line toolkit beyond the two-factor reset with
seven host-level commands: reset-password, create-emergency-admin,
clear-sessions, disable-sso, diagnostics, validate-db, and backup-data. Each
prints its result, exits with a meaningful status code, and writes an audit
entry where it changes state.

Document the toolkit in a new operator guide and link it from the recovery
and two-factor pages.

* feat(recovery): download the emergency command reference as a text file

The recovery commands are needed exactly when the dashboard is unreachable,
so reading them only in-app is a chicken-and-egg problem. Add a Download
button to the command-line section that saves the full
`docker compose exec sencho ...` reference as a text file, letting operators
keep it on hand before they need it. Reuses a shared download helper with the
existing diagnostics export.

* fix(recovery): harden diagnostics, backup, and emergency-admin against edge cases

Address findings from an independent review of the recovery toolkit:

- DiagnosticsService now degrades instead of throwing when a queried table is
  missing or corrupt: each read falls back and is folded into database.ok, so a
  broken database reports "problem detected" rather than failing the whole
  endpoint or showing a misleading healthy state with zeroed counts.
- backup-data refuses a destination that resolves to the live database, which
  would otherwise report success while producing no separate copy.
- create-emergency-admin now applies the same username rule as the user-
  management route, extracted to a shared helper so both stay in sync.

Adds tests for a missing read table, a malformed emergency-admin username, and
the backup same-target rejection.

backend/src/cli/backupData.ts

@@ -0,0 +1,57 @@
+/**
+ * Emergency CLI: back up the Sencho data directory (sencho.db + encryption.key)
+ * to a target directory. Uses SQLite's online backup so the copy is consistent
+ * even while Sencho is running.
+ *
+ * Run via:
+ *   docker compose exec sencho node dist/cli/backupData.js [destination-dir]
+ *
+ * With no argument it writes a timestamped folder under <DATA_DIR>/backups.
+ * Written to the audit log with actor `cli`.
+ */
+import fs from 'fs';
+import path from 'path';
+import { DatabaseService } from '../services/DatabaseService';
+import { auditCli, exitWith, type CliResult } from './_shared';
+
+function dataDir(): string {
+    return process.env.DATA_DIR || path.join(process.cwd(), 'data');
+}
+
+export async function backupData(destArg?: string): Promise<CliResult> {
+    const src = dataDir();
+    const keyPath = path.join(src, 'encryption.key');
+    const trimmedDest = destArg?.trim();
+    const dest = trimmedDest
+        ? path.resolve(trimmedDest)
+        : path.join(src, 'backups', `sencho-backup-${new Date().toISOString().replace(/[:.]/g, '-')}`);
+
+    const db = DatabaseService.getInstance();
+    // Refuse a destination that would write the copy onto the live database
+    // itself, which would report success while producing no separate backup.
+    if (path.resolve(dest, 'sencho.db') === path.resolve(db.getDb().name)) {
+        return { ok: false, message: 'Destination would overwrite the live database. Choose a different directory.' };
+    }
+
+    fs.mkdirSync(dest, { recursive: true });
+
+    // Online backup produces a consistent snapshot even while the DB is in use.
+    await db.getDb().backup(path.join(dest, 'sencho.db'));
+
+    let keyNote = '';
+    if (fs.existsSync(keyPath)) {
+        fs.copyFileSync(keyPath, path.join(dest, 'encryption.key'));
+        keyNote = ' + encryption.key';
+    }
+
+    auditCli(db, '/cli/backup-data', `CLI backed up data directory to ${dest}`);
+    return { ok: true, message: `Backup written to ${dest} (sencho.db${keyNote}). Store it somewhere safe; it contains your encryption key.` };
+}
+
+async function main(): Promise<void> {
+    exitWith(await backupData(process.argv[2]));
+}
+
+if (require.main === module) {
+    void main();
+}