feat(search): GOO-221 cursor/keyset pagination for SavedSearch alert listeners

All four alert code paths that previously loaded the entire SavedSearch
table into memory are replaced with bounded batch iteration backed by
the idx_savedsearch_alert_enabled partial index (merged in GOO-118).

Batch size is 500 rows; order-by is createdAt ASC, which matches the
index definition so the planner uses it for both the WHERE clause and
the cursor predicate.

Changed files:
- saved-search-alert.handler.ts: keyset loop on createdAt with
  alertEnabled=true, ALERT_BATCH_SIZE=500
- saved-search-alert-cron.service.ts: same pagination loop, removes
  the early-return on empty set (loop exits naturally on first empty page)
- residential-events.listener.ts: ResidentialPriceDropListener and
  ResidentialNewListingInProjectListener both paginated; select now
  includes createdAt to advance the cursor; shared ALERT_BATCH_SIZE

Tests:
- saved-search-alert.handler.spec.ts: adds createdAt to mock rows, adds
  3-page pagination test and orderBy/take assertion
- residential-events.listener.spec.ts: adds createdAt to mock rows, adds
  501-row pagination test verifying cursor advance on second call (9
  existing tests all pass)

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/modules/search/infrastructure/cron/saved-search-alert-cron.service.ts

@@ -4,10 +4,17 @@ import { Cron, CronExpression } from '@nestjs/schedule';
 import { SendNotificationCommand } from '@modules/notifications';
 import { PrismaService, LoggerService } from '@modules/shared';
 
+/** Rows processed per cursor-page. Aligns with idx_savedsearch_alert_enabled batch size. */
+const ALERT_BATCH_SIZE = 500;
+
 /**
  * Daily cron job that checks saved searches against new listings published since lastAlertAt.
  * This complements the real-time event-based handler by catching any listings that
  * were missed (e.g., due to service downtime or event processing failures).
+ *
+ * Memory footprint is bounded: rows are streamed in pages of {@link ALERT_BATCH_SIZE}
+ * via keyset pagination on `createdAt`, which the partial index
+ * `idx_savedsearch_alert_enabled` covers directly.
  */
 @Injectable()
 export class SavedSearchAlertCronService {
@@ -22,34 +29,49 @@ export class SavedSearchAlertCronService {
     this.logger.log('Starting daily saved search alert processing...', 'SavedSearchAlertCron');
 
     try {
-      const savedSearches = await this.prisma.savedSearch.findMany({
-        where: { alertEnabled: true },
-        include: {
-          user: { select: { id: true, email: true, fullName: true } },
-        },
-      });
-
-      if (savedSearches.length === 0) {
-        this.logger.log('No saved searches with alerts enabled', 'SavedSearchAlertCron');
-        return;
-      }
-
       let totalAlerts = 0;
+      let totalSearches = 0;
+      let cursor: Date | undefined;
 
-      for (const search of savedSearches) {
-        try {
-          const matchCount = await this.checkAndAlert(search);
-          totalAlerts += matchCount;
-        } catch (err) {
-          this.logger.warn(
-            `Failed to process alerts for saved search ${search.id}: ${err instanceof Error ? err.message : String(err)}`,
-            'SavedSearchAlertCron',
-          );
+      // Stream alert-enabled saved searches in bounded batches (keyset on createdAt).
+      // idx_savedsearch_alert_enabled covers WHERE alertEnabled = true ORDER BY createdAt.
+      do {
+        const batch = await this.prisma.savedSearch.findMany({
+          where: {
+            alertEnabled: true,
+            ...(cursor ? { createdAt: { gt: cursor } } : {}),
+          },
+          include: {
+            user: { select: { id: true, email: true, fullName: true } },
+          },
+          orderBy: { createdAt: 'asc' },
+          take: ALERT_BATCH_SIZE,
+        });
+
+        if (batch.length === 0) break;
+
+        totalSearches += batch.length;
+
+        for (const search of batch) {
+          try {
+            const matchCount = await this.checkAndAlert(search);
+            totalAlerts += matchCount;
+          } catch (err) {
+            this.logger.warn(
+              `Failed to process alerts for saved search ${search.id}: ${err instanceof Error ? err.message : String(err)}`,
+              'SavedSearchAlertCron',
+            );
+          }
         }
-      }
+
+        // Advance cursor to the last row's createdAt for the next page.
+        cursor = batch[batch.length - 1]!.createdAt;
+
+        if (batch.length < ALERT_BATCH_SIZE) break;
+      } while (true);
 
       this.logger.log(
-        `Daily saved search alert processing completed: ${totalAlerts} alerts sent for ${savedSearches.length} searches`,
+        `Daily saved search alert processing completed: ${totalAlerts} alerts sent for ${totalSearches} searches`,
         'SavedSearchAlertCron',
       );
     } catch (err) {