fix(notification): Paginate and bulk-insert follower fan-out in notifyFollowers()

[Copilot]

notifyFollowers() loaded the entire follower list into memory and issued one INSERT per follower inside a single async task — an O(n) memory and O(n) DB round-trip problem that degrades badly at scale.

Changes

• PostNotificationEventListener: Replace unbounded findFollowersByFollowee() with a paginated do-while loop (FOLLOWER_BATCH_SIZE = 100); skip empty pages to avoid unnecessary DB calls
• NotificationService: Add createNotifications(List<CreateNotificationDTO>) using saveAll() for a single bulk insert per page, with per-batch SSE event publishing
• UserFollowRepository: Remove the non-paginated findFollowersByFollowee(): List<User> query (was the sole source of the unbounded load)

// Before: load all N followers, N individual INSERTs
val followers = userFollowRepository.findFollowersByFollowee(writer)
followers.forEach { notificationService.createNotification(createRequest(it)) }

// After: paginate in batches of 100, 1 bulk INSERT per page
do {
    val page = userFollowRepository.findFollowersByFolloweeOrderByIdDesc(writer, PageRequest.of(pageNumber, FOLLOWER_BATCH_SIZE))
    if (page.content.isNotEmpty()) {
        notificationService.createNotifications(page.content.map { createRequest(it) })
    }
    pageNumber++
} while (page.hasNext())

Tests

• NotificationServiceTest: adds CreateNotificationsTest covering bulk save and event publishing
• PostNotificationEventListenerTest: new test class covering single-page fan-out, multi-page pagination, entity-not-found early exit, and empty-follower no-op

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• api.foojay.io
  • Triggering command: /usr/lib/jvm/temurin-25-jdk-amd64/bin/java /usr/lib/jvm/temurin-25-jdk-amd64/bin/java --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.lang.invoke=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-opens=java.base/java.nio.charset=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.util.concurrent=ALL-UNNAMED --add-opens=java.base/java.util.concurrent.atomic=ALL-UNNAMED --add-opens=java.xml/javax.xml.namespace=ALL-UNNAMED --add-opens=java.base/java.time=ALL-UNNAMED --enable-native-access=ALL-UNNAMED -XX:MaxMetaspaceSize=384m -XX:+HeapDumpOnOutOfMemoryError -Xms256m -Xmx512m (dns block)
  • Triggering command: /tmp/java24/bin/java /tmp/java24/bin/java --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.lang.invoke=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-opens=java.base/java.nio.charset=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.util.concurrent=ALL-UNNAMED --add-opens=java.base/java.util.concurrent.atomic=ALL-UNNAMED --add-opens=java.xml/javax.xml.namespace=ALL-UNNAMED --add-opens=java.base/java.time=ALL-UNNAMED --enable-native-access=ALL-UNNAMED -XX:MaxMetaspaceSize=384m -XX:+HeapDumpOnOutOfMemoryError -Xms256m -Xmx512m (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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