chore: remove Shoutrrr residue and dead notification legacy code
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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