fix(config-history): only auto-snapshot devices with SSH configured; add settings UI (v0.17.4-beta)

Resolves #86 — MikroTik Manager was attempting SSH connections to devices that
have no SSH credentials configured, flooding RouterOS logs with failed logins.

Cause: config snapshots (a separate feature from scheduled backups, enabled by
default, hourly) capture `/export` over SSH via BackupService.exportConfig, which
silently falls back to the API credentials for the SSH login. On devices where
SSH was never set up, that auth fails every interval (and on each restart), and
there was no UI switch to turn it off.

- PollerService.processConfigSnapJob now skips devices without SSH credentials,
  so no SSH connection is attempted where SSH isn't configured. Manual "Capture
  snapshot" / backup actions are unchanged (user-initiated, with error feedback).
- Add a "Configuration Snapshots" section to Settings → General: enable toggle,
  interval, and retention (the feature was on-by-default with no UI control),
  with a note that it runs over SSH and only on devices that have SSH creds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/pages/SettingsPage.tsx

@@ -667,6 +667,64 @@ export default function SettingsPage() {
             </div>
           </div>
 
+          {/* Configuration Snapshots */}
+          <div className="card p-5">
+            <h3 className="font-semibold text-gray-900 dark:text-white mb-1">Configuration Snapshots</h3>
+            <p className="text-xs text-gray-400 dark:text-slate-500 mb-4">
+              Periodically captures each device&apos;s <code className="font-mono">/export</code> for the Config History / drift detection feature.
+              This runs <strong>over SSH</strong>, so it only executes on devices that have <strong>SSH credentials configured</strong> — devices without them are skipped (no connection is attempted).
+            </p>
+            <div className="space-y-3">
+              <div className="flex items-center justify-between">
+                <div className="text-sm font-medium text-gray-700 dark:text-slate-300">Enable config snapshots</div>
+                <button
+                  onClick={() => isAdmin && updateSettingsMutation.mutate({ config_snapshot_enabled: settings['config_snapshot_enabled'] === false })}
+                  disabled={!isAdmin}
+                  className={clsx(
+                    'relative inline-flex h-6 w-11 flex-shrink-0 rounded-full border-2 border-transparent transition-colors duration-200',
+                    isAdmin ? 'cursor-pointer' : 'cursor-not-allowed opacity-50',
+                    settings['config_snapshot_enabled'] !== false ? 'bg-blue-600' : 'bg-gray-300 dark:bg-slate-600'
+                  )}
+                >
+                  <span className={clsx(
+                    'inline-block h-5 w-5 transform rounded-full bg-white shadow transition duration-200',
+                    settings['config_snapshot_enabled'] !== false ? 'translate-x-5' : 'translate-x-0'
+                  )} />
+                </button>
+              </div>
+              <div className="flex items-center justify-between gap-4">
+                <div>
+                  <div className="text-sm font-medium text-gray-700 dark:text-slate-300">Snapshot interval</div>
+                  <div className="text-xs text-gray-400">minutes</div>
+                </div>
+                <input
+                  type="number"
+                  className="input w-24 text-center disabled:opacity-50 disabled:cursor-not-allowed"
+                  value={settings['config_snapshot_interval_min'] as number ?? 60}
+                  onChange={(e) => updateSettingsMutation.mutate({ config_snapshot_interval_min: parseInt(e.target.value) })}
+                  min="15"
+                  step="15"
+                  disabled={!isAdmin || settings['config_snapshot_enabled'] === false}
+                />
+              </div>
+              <div className="flex items-center justify-between gap-4">
+                <div>
+                  <div className="text-sm font-medium text-gray-700 dark:text-slate-300">Retention</div>
+                  <div className="text-xs text-gray-400">snapshots kept per device</div>
+                </div>
+                <input
+                  type="number"
+                  className="input w-24 text-center disabled:opacity-50 disabled:cursor-not-allowed"
+                  value={settings['config_snapshot_retention'] as number ?? 30}
+                  onChange={(e) => updateSettingsMutation.mutate({ config_snapshot_retention: parseInt(e.target.value) })}
+                  min="1"
+                  step="1"
+                  disabled={!isAdmin || settings['config_snapshot_enabled'] === false}
+                />
+              </div>
+            </div>
+          </div>
+
           {/* Reverse DNS */}
           <div className="card p-5">
             <h3 className="font-semibold text-gray-900 dark:text-white mb-1">Reverse DNS Lookup</h3>

frontend/src/version.ts

@@ -1 +1 @@
-export const APP_VERSION = 'v0.17.3 Beta';
+export const APP_VERSION = 'v0.17.4 Beta';