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/validateDb.ts

@@ -0,0 +1,48 @@
+/**
+ * Emergency CLI: validate that the database and encryption key are intact.
+ * Runs a SQLite integrity check, confirms the core tables exist, confirms the
+ * encryption key is present and usable, and confirms at least one admin exists.
+ * Exits non-zero if any check fails, so it can gate a restore decision in a
+ * script. Read-only.
+ *
+ * Run via:
+ *   docker compose exec sencho node dist/cli/validateDb.js
+ */
+import { collectDiagnostics } from '../services/DiagnosticsService';
+import { exitWith, type CliResult } from './_shared';
+
+export async function validateDb(): Promise<CliResult> {
+    const report = await collectDiagnostics();
+    const problems: string[] = [];
+
+    if (!report.database.ok) {
+        problems.push(`database integrity: ${report.database.integrity}`);
+        if (report.database.missingTables.length > 0) {
+            problems.push(`missing core tables: ${report.database.missingTables.join(', ')}`);
+        }
+    }
+    if (!report.encryptionKey.present) {
+        problems.push('encryption.key is missing');
+    } else if (!report.encryptionKey.valid) {
+        problems.push('encryption.key is present but invalid');
+    }
+    if (report.auth.adminCount === 0) {
+        problems.push('no admin users exist');
+    }
+
+    if (problems.length > 0) {
+        return { ok: false, message: `Validation failed:\n  - ${problems.join('\n  - ')}` };
+    }
+    return {
+        ok: true,
+        message: `Database OK (integrity ok, ${report.auth.adminCount} admin(s), encryption key valid).`,
+    };
+}
+
+async function main(): Promise<void> {
+    exitWith(await validateDb());
+}
+
+if (require.main === module) {
+    void main();
+}