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/api/instruments/contracts/route.ts

@@ -1,4 +1,4 @@
-import { NextResponse } from 'next/server';
+import { NextRequest, NextResponse } from 'next/server';
 import { getClients } from '@/lib/clients';
 import { getInstruments } from '@/lib/db';
 import { resolveContracts, getAllCachedContracts } from '@/lib/contract-resolver';
@@ -13,10 +13,11 @@ export async function GET() {
 
 /**
  * POST /api/instruments/contracts
- * Triggers a fresh resolve of all enabled instruments via Tradovate + yfinance.
- * Returns the resolved contracts with volume data.
+ * Triggers a fresh resolve via Tradovate + Yahoo Finance.
+ * Body (optional): { symbols: string[] } — when provided, resolves only those symbols.
+ * Otherwise resolves all enabled instruments.
  */
-export async function POST() {
+export async function POST(req: NextRequest) {
     try {
         const clients = getClients();
         // Find first client with a valid token
@@ -29,8 +30,12 @@ export async function POST() {
             return NextResponse.json({ error: 'No authenticated client available' }, { status: 503 });
         }
 
-        const instruments = getInstruments().filter((i) => i.enabled);
-        const symbols = instruments.map((i) => i.symbol);
+        const body = await req.json().catch(() => ({}));
+        const requestedSymbols: string[] | undefined = Array.isArray(body?.symbols) ? body.symbols : undefined;
+
+        const symbols = requestedSymbols?.length
+            ? requestedSymbols
+            : getInstruments().filter((i) => i.enabled).map((i) => i.symbol);
 
         const resolved = await resolveContracts(symbols, accessToken);
         return NextResponse.json(resolved);