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/diagnostics.ts

@@ -0,0 +1,29 @@
+/**
+ * Emergency CLI: print a redacted diagnostic summary (version, database
+ * integrity, encryption-key status, admin/SSO/MFA counts, non-secret config) as
+ * JSON. Read-only; carries no secrets (see DiagnosticsService for the
+ * redaction allowlist). Safe to copy into a bug report.
+ *
+ * Run via:
+ *   docker compose exec sencho node dist/cli/diagnostics.js
+ */
+import { collectDiagnostics } from '../services/DiagnosticsService';
+
+export async function printDiagnostics(): Promise<void> {
+    const report = await collectDiagnostics();
+    console.log(JSON.stringify(report, null, 2));
+}
+
+async function main(): Promise<void> {
+    try {
+        await printDiagnostics();
+        process.exit(0);
+    } catch (err) {
+        console.error(`Failed to collect diagnostics: ${(err as Error).message}`);
+        process.exit(1);
+    }
+}
+
+if (require.main === module) {
+    void main();
+}