fix: add required filters for billing items and validate task project references

- Add buildBillingItemsFilter() to provide required itemDate filter
  BillingItems API requires a filter parameter and returns 500 error without it
- Add validation for task project_id foreign key references
  Tasks with invalid project_id now have the field nullified instead of failing
- Add getValidProjectIds() method to fetch valid project IDs from database

Fixes:
- Billing Items: 'Value cannot be null. Parameter name: filters' error
- Tasks: 'violates foreign key constraint tasks_project_id_fkey' error

Both entities should now sync successfully.

lib/utils/sync-helpers.ts

@@ -332,6 +332,30 @@ export function buildTimeEntriesFilter(yearsBack: number = 2): Array<{ field: st
   ];
 }
 
+/**
+ * Build special filter for billing items (requires filter)
+ * @param yearsBack Number of years to look back (default: 2)
+ * @returns Query filter array for billing items
+ */
+export function buildBillingItemsFilter(yearsBack: number = 2): Array<{ field: string; op: string; value: any }> {
+  // BillingItems API requires a filter. Use itemDate to limit the range
+  // Calculate date from X years ago
+  const cutoffDate = new Date();
+  const millisecondsPerYear = 365.25 * 24 * 60 * 60 * 1000;
+  const millisecondsBack = yearsBack * millisecondsPerYear;
+  cutoffDate.setTime(cutoffDate.getTime() - millisecondsBack);
+
+  console.log(`BillingItems filter: itemDate >= ${cutoffDate.toISOString()} (${yearsBack} years back)`);
+
+  return [
+    {
+      field: 'itemDate',
+      op: 'gte',
+      value: cutoffDate.toISOString(),
+    },
+  ];
+}
+
 /**
  * Calculate estimated sync duration based on record count
  * @param recordCount Number of records to sync