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.

.gitignore

@@ -78,6 +78,11 @@ backend/node_modules/
 backend/package.json
 backend/package-lock.json
 
+# Root-level artifact files (non-documentation)
+FIREFOX_E2E_FIXES_SUMMARY.md
+verify-security-state-for-ui-tests
+categories.txt
+
 # -----------------------------------------------------------------------------
 # Databases
 # -----------------------------------------------------------------------------
@@ -297,6 +302,7 @@ docs/plans/current_spec_notes.md
 tests/etc/passwd
 trivy-image-report.json
 trivy-fs-report.json
+trivy-report.json
 backend/# Tools Configuration.md
 docs/plans/requirements.md
 docs/plans/design.md