fix(security): harden outbound client configuration in notification senders

[Wikid82]

Summary

Hardens the outbound client configuration used by the notification senders so the production path no longer enables a testing-only allowance. The allowance now sits behind an unexported test-only seam shared by both senders, which also removes the duplicated client construction.

Behaviour note

Notification webhooks can no longer target loopback addresses (localhost, 127.0.0.1, ::1). A receiver running on the same host must be reached via a non-loopback address (LAN IP or Docker hostname). Documented in docs/features/notifications.md.

Testing

• Regression tests cover default rejection and the test-only seam
• go test -race ./internal/services/..., go test ./internal/api/..., make lint-fast pass
• Backend coverage 92.3%; patch coverage 100%
• QA report: docs/reports/qa_report.md

Built with help from Claude Code.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

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📦 SBOM Summary

• Components: 1870

🔍 Vulnerability Scan

Severity	Count
🔴 Critical	0
🟠 High	0
🟡 Medium	0
🟢 Low	0
Total	0

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