Remove all deprecated Shoutrrr integration artifacts and dead legacy fallback
code from the notification subsystem.
- Remove legacySendFunc field, ErrLegacyFallbackDisabled error, and
legacyFallbackInvocationError() from notification service
- Delete ShouldUseLegacyFallback() from notification router; simplify
ShouldUseNotify() by removing now-dead providerEngine parameter
- Remove EngineLegacy engine constant; EngineNotifyV1 is the sole engine
- Remove legacy.fallback_enabled feature flag, retiredLegacyFallbackEnvAliases,
and parseFlagBool/resolveRetiredLegacyFallback helpers from flags handler
- Remove orphaned EmailRecipients field from NotificationConfig model
- Delete feature_flags_coverage_v2_test.go (tested only the retired flag path)
- Delete security_notifications_test.go.archived (stale archived file)
- Move FIREFOX_E2E_FIXES_SUMMARY.md to docs/implementation/
- Remove root-level scan artifacts tracked in error; add gitignore patterns to
prevent future tracking of trivy-report.json and related outputs
- Update ARCHITECTURE.instructions.md: Notifications row Shoutrrr → Notify
No functional changes to active notification dispatch or mail delivery.
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