feat(notif): add digest mode to batch notifications into a single message per scan run - #1813

What this does

Adds a digest boolean field to the Notif config that, when enabled, buffers all image update notifications from a single scan run and fires them as one batched message instead of one message per image.

This is useful when monitoring many containers across multiple hosts -- without digest mode, a single scan can flood a Slack channel (or other notifier) with dozens of individual messages.

How it works

• A pendingNotify flag is set on each NotifEntry that passes all notification filters in job.go, instead of immediately calling Send
• After all workers finish (wg.Wait() in app.go), SendBatch is called once with the full NotifEntries set
• Notifiers that want custom batch formatting implement the new optional BatchHandler interface (SendBatch(*model.NotifEntries) error)
• Notifiers that don't implement BatchHandler fall back to individual Send calls for each pending entry, so existing notifiers work without changes

Changes

Model / core

• internal/model/notif.go — adds Digest bool to Notif; adds pendingNotify field with MarkPendingNotify()/PendingNotify() accessors to NotifEntry; adds mutex-safe Add() to NotifEntries
• internal/notif/notifier/notifier.go — adds optional BatchHandler interface
• internal/notif/client.go — adds IsDigest() and SendBatch()
• internal/app/job.go — marks entry pending instead of sending immediately when digest mode is on
• internal/app/app.go — calls SendBatch after wg.Wait() when digest mode is on

Slack notifier

• internal/notif/slack/client.go — implements BatchHandler.SendBatch with a compact bullet-list summary (registry-stripped image names, HubLink hyperlinks where available, build date, short sha256 digest, clean hostname)

Config

notif:
  digest: true
  slack:
    webhookURL: https://hooks.slack.com/services/...

Or via environment variable:

DIUN_NOTIF_DIGEST=true

Example output (Slack)

2 image(s) need attention — 0 new, 2 updated (host: docker-host-02)
• <https://hub.docker.com/r/library/nginx|library/nginx:latest> updated · Jul 17 · `sha256:1c4e1721`
• `searxng/searxng:latest` updated · Jul 11 · `sha256:8a865b0a`

Notes

• Only the Slack notifier implements BatchHandler in this PR; other notifiers fall back to individual sends automatically
• The NotifEntries.mu mutex guards concurrent worker writes; the pointer receiver on SendBatch avoids copying it
• No breaking changes to existing config or notifier interfaces

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