Add passphrase-protected export/import for integrations, DNS providers, and settings

Nothing let you back up or migrate the app's own configuration short
of copying the raw SQLite file. Adds Settings -> Backup: export
decrypts every integration/DNS provider credential (normally
encrypted at rest with this server's CREDENTIALS_ENCRYPTION_KEY) and
re-encrypts the whole payload with a passphrase you choose (scrypt-
derived key, AES-256-GCM), so the file is portable to a different
instance with a different encryption key rather than being tied to
this one. Import decrypts with that passphrase and merges settings
onto the current ones; integrations/DNS providers are only added when
no existing row shares their type+name, so re-running an import never
duplicates or overwrites a working credential.

Scope is configuration only — no DNS records, secrets, IPAM, servers,
or audit/diagnostic log data.

Verified end-to-end against two isolated scratch databases with
different encryption keys (proving actual cross-instance portability,
not just round-tripping through the same key): export -> encrypt ->
write file -> decrypt on the other DB -> import -> re-decrypt the
newly created integration/provider using the target's own key,
confirming the plaintext credentials survived correctly; a wrong
passphrase failed loudly (GCM auth failure) as expected; and
re-running the same import a second time skipped both rows instead of
duplicating them. Confirmed the real dev database's mtime was
untouched throughout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/App.tsx

@@ -24,6 +24,7 @@ import BadgeSettings from "./pages/settings/BadgeSettings";
 import DisplaySettings from "./pages/settings/DisplaySettings";
 import CacheSettings from "./pages/settings/CacheSettings";
 import LogSettings from "./pages/settings/LogSettings";
+import BackupSettings from "./pages/settings/BackupSettings";
 import AppShell from "./layout/AppShell";
 import { setDateTimeSettings } from "./utils/date";
 import { setPageSize } from "./utils/pageSize";
@@ -128,6 +129,7 @@ export default function App() {
           <Route path="display" element={<DisplaySettings />} />
           <Route path="cache" element={<CacheSettings />} />
           <Route path="logs" element={<LogSettings />} />
+          <Route path="backup" element={<BackupSettings />} />
         </Route>
       </Routes>
     </AppShell>