Auto-fetch contract on symbol enable; hide contract when disabled

- POST /api/instruments/contracts now accepts optional { symbols[] } body
  to resolve a subset rather than all enabled instruments
- Settings toggle() fires a targeted resolve when enabling a symbol,
  merging the result into contracts state without a full page refresh
- Active Contract cell is hidden (null) when the instrument is disabled

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/settings/page.tsx

@@ -67,6 +67,22 @@ export default function SettingsPage() {
             headers: { 'Content-Type': 'application/json' },
             body: JSON.stringify({ enabled }),
         });
+
+        // When enabling a symbol, auto-resolve its active contract
+        if (enabled) {
+            fetch('/api/instruments/contracts', {
+                method: 'POST',
+                headers: { 'Content-Type': 'application/json' },
+                body: JSON.stringify({ symbols: [symbol] }),
+            })
+                .then((r) => r.json())
+                .then((fresh: Record<string, ResolvedContract | null>) => {
+                    if (!fresh.error) {
+                        setContracts((prev) => ({ ...prev, ...fresh }));
+                    }
+                })
+                .catch(() => {});
+        }
     }
 
     async function saveSettings() {
@@ -161,7 +177,7 @@ export default function SettingsPage() {
                                                 {instr.symbol}
                                             </td>
                                             <td className="px-4 py-2.5">
-                                                {c ? (
+                                                {instr.enabled && c ? (
                                                     <div className="flex items-center gap-2">
                                                         <span className={`font-mono text-sm ${wasRolled ? 'text-amber-600 font-semibold' : 'text-slate-600'}`}>
                                                             {c.name}
@@ -177,9 +193,7 @@ export default function SettingsPage() {
                                                             </span>
                                                         )}
                                                     </div>
-                                                ) : (
-                                                    <span className="text-xs text-slate-300 italic">—</span>
-                                                )}
+                                                ) : null}
                                             </td>
                                             <td className="px-4 py-2.5 text-right">
                                                 <button