Formatted Slack sends bypass secret scrubbing in metadata fields

[Calmingstorm]

Failure scenario

1. Enable the Slack notifier with scrub_secrets=true.
2. Call the formatted send path with a normal title/message but put credential-shaped text in source or severity (the Web API accepts both fields from the request body).
3. Inspect the JSON posted to Slack.

send_formatted() scrubs only title and message. build_formatted_payload() copies source and uppercases severity into attachment fields unchanged. send() then sees a custom payload and scrubs only its top-level text, not nested attachment fields. The credential-shaped metadata is therefore transmitted despite secret scrubbing being enabled.

The same nested-payload behavior affects any caller of send(..., payload=...): secrets in blocks or attachments are not scrubbed.

Sites

• src/notifications/slack.py:57-61 copies source and severity into nested fields.
• src/notifications/slack.py:151-155 scrubs only top-level text for custom payloads.
• src/notifications/slack.py:185-192 leaves formatted metadata unscrubbed.
• src/web/api/integrations.py:498-505 supplies request-controlled severity and source to this path.

Expected result

Scrub every user-controlled string in the complete outgoing Slack payload, including nested attachments/blocks and formatted metadata, before posting it.

[Calmingstorm]

The Slack integration was removed in v4.7.0 (PR #367), so this no longer applies. Generic Slack-token redaction remains active.