[liangyanbin126]

What type of PR is this?

• Bug fix
• New feature
• Refactor / docs / chore / tests (no behavior change)
  What this PR does / why we need it:
  Support dashboard screenshots in FlashDuty alert notifications so responders can view metric trends directly in alerts.
• Upload screenshots before sending alerts and attach the returned image key.
• Reuse uploaded images across FlashDuty channels within the same notification batch.
• Continue sending alerts if screenshot uploads fail.
• Preserve original event data for other notification channels and remove base64 screenshots from outgoing FlashDuty events.
• Add automated tests for upload failures, multi-channel reuse, event preservation, image size limits, and rule configuration precision.
  Which issue(s) this PR fixes:
  Fixes Support dashboard screenshots in FlashDuty alert notifications #3446
  Evidence (REQUIRED for bug fixes and features):
  Reproduction steps:

1. Configure a FlashDuty notification channel.
2. Send an alert event containing a dashboard screenshot in ShotImageBase64.
3. Send the same batch to multiple FlashDuty channels.
4. Repeat with a failed image upload to verify that alerts still get sent.
   Before:
   FlashDuty notifications did not upload screenshots or attach the corresponding image references.
   After (or the new feature page):
   Screenshots are uploaded before notification delivery. Outgoing alerts include annotations.img_key and rule_config.snapshot_pic_url, and multiple channels reuse the uploaded image. Upload failures do not block alert delivery.
   Local automated verification passed:
   go test ./alert/sender/provider -count=1
   go test -race ./alert/sender/provider -run FlashDuty -count=1
   git diff --check
   Real-instance verification and screenshots are still pending. Full-repository build and tests have not been run.
   Checklist:

• I ran this change on a real Nightingale instance myself and attached screenshots above.
• go build ./... and go test ./... pass locally.
• If integrations/** changed: go run ./cmd/integrations-alert-action check and go run ./cmd/integrations-i18n check -scope all pass. — Not applicable.
• If DB schema changed: gorm struct AND docker/initsql/a-n9e.sql, docker/sqlite.sql, docker/migratesql/migrate.sql are updated. — Not applicable.
• AI tools were used to write this PR: yes — OpenAI Codex assisted with tests and the PR description.
  Special notes for your reviewer:
  This change handles screenshots already present in alert events; it does not enable upstream screenshot generation. Each event attaches one screenshot, selected by sorted key order after skipping blank values. Decoded images larger than 5 MiB are rejected. Image reuse is scoped to a single Notify call.