fix: performance hardening — eliminate full table scans (#9)

Inventory:
- /issues: replaced full scan + client filter with NocoDB server-side
  WHERE filter (Received eq Issues/Issue). Single query, ~200 rows max.
- /needs-review-count: replaced full scan with server-side WHERE +
  limit=1 + pageInfo.totalRows. Returns count without fetching data.

Budget:
- buildLookups(): added 2-minute cache for payee/account/category maps.
  Eliminates 3 API calls per request for repeated queries.
- /summary cache (added earlier): 1-minute TTL still active.

Files: services/inventory/server.js, services/budget/server.js

services/budget/server.js

@@ -102,8 +102,12 @@ function currentMonth() {
   return `${now.getFullYear()}-${String(now.getMonth() + 1).padStart(2, '0')}`;
 }
 
-/** Build lookup maps for payees, accounts, and categories. */
+/** Build lookup maps for payees, accounts, and categories (cached 2 min). */
+let _lookupsCache = { data: null, expiresAt: 0 };
 async function buildLookups() {
+  if (_lookupsCache.data && Date.now() < _lookupsCache.expiresAt) {
+    return _lookupsCache.data;
+  }
   const [payees, accounts, categories] = await Promise.all([
     api.getPayees(),
     api.getAccounts(),
@@ -115,7 +119,9 @@ async function buildLookups() {
   for (const a of accounts) accountMap[a.id] = a.name;
   const categoryMap = {};
   for (const c of categories) categoryMap[c.id] = c.name;
-  return { payeeMap, accountMap, categoryMap };
+  const result = { payeeMap, accountMap, categoryMap };
+  _lookupsCache = { data: result, expiresAt: Date.now() + 120000 };
+  return result;
 }
 
 /** Enrich a transaction with resolved names. */