Parallel scrape workers (proxy-gated, configurable)

Fan a scrape run out across N workers, each with its own DB session and
its own scraper backend — and since every backend picks a random sticky
session id, each worker scrapes from a distinct residential IP. Dates are
interleaved across workers so each covers a spread of the range. Cuts a
150-date batch from ~30-45 min to ~12-15 min at 3 workers.

- booking_scraper_concurrency config key (default 3); Settings → Scraper
  Proxy has a "Parallel workers" field
- Forced to 1 when the proxy is off (N workers would share one IP and
  hammer it) or when there's a single date
- Both manual and queue paths routed through _scrape_dates_concurrent;
  workers mark their own queue items
- Per-worker rotate-on-block replaces the old global pause-on-block

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

frontend/src/pages/Settings.tsx

@@ -123,6 +123,12 @@ function ProxyTab() {
   const [password, setPassword] = useState('')
   const [testResult, setTestResult] = useState<ProxyTestResult | null>(null)
   const [testError, setTestError] = useState<string | null>(null)
+  const [workers, setWorkers] = useState('3')
+
+  const { data: sysCfg } = useQuery<SystemConfig>({
+    queryKey: ['system-config'],
+    queryFn: () => api.get('/competitors/config/system').then(r => r.data),
+  })
 
   // Seed form once the stored config arrives (password stays blank by design)
   useEffect(() => {
@@ -135,6 +141,17 @@ function ProxyTab() {
     }
   }, [cfg])
 
+  useEffect(() => {
+    if (sysCfg?.booking_scraper_concurrency) setWorkers(String(sysCfg.booking_scraper_concurrency))
+  }, [sysCfg])
+
+  const saveWorkers = useMutation({
+    mutationFn: () => api.post('/competitors/config/system', {
+      key: 'booking_scraper_concurrency', value: String(Math.max(1, parseInt(workers) || 1)),
+    }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: ['system-config'] }),
+  })
+
   const save = useMutation({
     mutationFn: () =>
       api.post('/competitors/config/proxy', {
@@ -248,6 +265,38 @@ function ProxyTab() {
           )}
         </div>
       </div>
+
+      <div className="card">
+        <div className="card-header">
+          <span style={{ display: 'flex', alignItems: 'center', gap: 8 }}>
+            <Network size={16} strokeWidth={1.75} />
+            Parallel Workers
+          </span>
+        </div>
+        <div className="card-body" style={{ display: 'flex', flexDirection: 'column', gap: 12 }}>
+          <p style={{ fontSize: 13, color: 'var(--text-mid)', margin: 0 }}>
+            How many dates to scrape at once. Each worker runs its own browser on its own
+            residential IP, so a run finishes roughly this many times faster. Only takes effect
+            when the proxy is enabled (without it, all workers would share one IP). Budget about
+            0.4&nbsp;GB of RAM per worker — 3 is safe on the current 4&nbsp;GB; raise it after
+            adding RAM.
+          </p>
+          <div style={{ display: 'flex', gap: 8, alignItems: 'center' }}>
+            <input type="number" min={1} max={12} value={workers}
+              onChange={e => setWorkers(e.target.value)} style={{ width: 90 }} />
+            <button className="btn btn-primary btn-sm" onClick={() => saveWorkers.mutate()}
+              disabled={saveWorkers.isPending}>
+              <Save size={13} strokeWidth={1.75} />
+              {saveWorkers.isPending ? 'Saving…' : 'Save'}
+            </button>
+            {!enabled && (
+              <span style={{ fontSize: 12, color: 'var(--warning)' }}>
+                Enable the proxy above for this to apply
+              </span>
+            )}
+          </div>
+        </div>
+      </div>
     </div>
   )
 }
@@ -493,6 +542,7 @@ function SystemTab({ config, isLoading }: { config: SystemConfig | undefined; is
     'booking_proxy_enabled',
     'booking_proxy_host',
     'booking_proxy_country',
+    'booking_scraper_concurrency',
     'sync_newbook_current_rates_enabled',
     'sync_newbook_current_rates_time',
   ]