fix(ai-budgets): tighten rollover period filter to exclude stale entries

- Previous filter only checked e.recordedAt < currentPeriodStart
- Now also checks e.recordedAt >= prevPeriodStart (lower bound)
- Prevents entries from periods before the previous one from inflating
  the spent amount, which would reduce the rollover incorrectly
- 12 ai-budgets tests passing

services/platform-service/src/modules/ai-budgets/routes.ts

@@ -330,13 +330,14 @@ export async function aiBudgetRoutes(app: FastifyInstance) {
     }
     const prevPeriodStart = toIsoDayBoundary(periodStart(prev, policy.period));
 
-    // Get spend from previous period
+    // Get spend from previous period only (bounded to prev→current)
     const prevEntries = await repo.listSpendEntries(access.productId, {
       policyId: id,
       since: prevPeriodStart,
     });
-    // Only count entries in previous period (before current period)
-    const prevPeriodEntries = prevEntries.filter(e => e.recordedAt < currentPeriodStart);
+    const prevPeriodEntries = prevEntries.filter(
+      e => e.recordedAt >= prevPeriodStart && e.recordedAt < currentPeriodStart
+    );
     const spentUsd = prevPeriodEntries.reduce((sum, e) => sum + e.costUsd, 0);
     const remainingUsd = Math.max(0, policy.budgetUsd - spentUsd);
     const rolledOverUsd = remainingUsd; // Full rollover — could add a cap later