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.
- Added tests to `proxyhost_service_validation_test.go` to validate fallback parsing and handle invalid hostname characters.
- Introduced new tests for DNS challenge validation in `proxyhost_service_validation_test.go`.
- Updated `current_spec.md` to reflect changes in testing strategy and coverage goals for PR #729.
- Enhanced `Security.functional.test.tsx` to include navigation test for Notifications button.
- Mocked `useNavigate` from `react-router-dom` to verify navigation behavior in Security page tests.
- Added validation to reject non-discord provider types in create, update, test, and preview operations.
- Updated the notifications form to automatically normalize non-discord types to discord.
- Modified UI to display explicit messaging for deprecated and non-dispatch statuses for non-discord providers.
- Enhanced tests to cover new validation logic and UI changes for provider types.