feat(alertd): KAM-483: re-implement slack integration via webhook targets

[passcod]

Summary

Re-implements Slack integration in alertd. Slack targets are configured in _targets.yml with a webhook URL and optional custom fields. Multiple target types can share the same ID, so an alert can notify both email and Slack simultaneously.

Context

KAM-483 — Slack alerts were removed along with Zendesk in an earlier simplification, but it turns out at least one deployment (Aspen) relied on Slack alerts because email alerts to Zendesk didn't work properly. This caused confusion where alerts appeared to be working (email) while support couldn't see them (Slack).

Changes

• targets/slack.rs: New TargetSlack struct with webhook URL and configurable fields, adapted from the legacy bestool tamanu alerts implementation
• targets.rs: Introduce TargetConnection enum (Email | Slack) replacing the email-only ExternalTarget.conn; target type is auto-detected from YAML fields (addresses → email, webhook → slack)
• alert.rs / daemon.rs: Add http_client: reqwest::Client to InternalContext for webhook POSTs
• scheduler.rs / events.rs: Pass HTTP client through to ResolvedTarget::send()
• Remove facet dependency: Was only used in derives, not needed
• Add url crate with serde feature for webhook URL parsing
• Documentation: Updated TARGETS.md and ALERTS.md

No breaking changes

Existing email-only _targets.yml configurations continue to work unchanged. Slack is purely additive.

Example _targets.yml

targets:
  # Email and Slack with the same ID — alerts go to both
  - id: ops-team
    addresses:
      - ops@example.com

  - id: ops-team
    webhook: https://hooks.slack.com/services/T.../B.../xxx