refactor(semantics): migrate legacy @TIER to @COMPLEXITY annotations

- Replaced @TIER: TRIVIAL with @COMPLEXITY: 1
- Replaced @TIER: STANDARD with @COMPLEXITY: 3
- Replaced @TIER: CRITICAL with @COMPLEXITY: 5
- Manually elevated specific critical/complex components to levels 2 and 4
- Ignored legacy, specs, and node_modules directories
- Updated generated semantic map

backend/src/services/notifications/__tests__/test_notification_service.py

@@ -1,5 +1,5 @@
 # [DEF:backend.src.services.notifications.__tests__.test_notification_service:Module]
-# @TIER: STANDARD
+# @COMPLEXITY: 3
 # @PURPOSE: Unit tests for NotificationService routing and dispatch logic.
 
 import pytest

backend/src/services/notifications/providers.py

@@ -1,6 +1,6 @@
 # [DEF:backend.src.services.notifications.providers:Module]
 #
-# @TIER:      CRITICAL
+# @COMPLEXITY: 5
 # @SEMANTICS: notifications, providers, smtp, slack, telegram, abstraction
 # @PURPOSE:   Defines abstract base and concrete implementations for external notification delivery.
 # @LAYER:     Infra

backend/src/services/notifications/service.py

@@ -1,6 +1,6 @@
 # [DEF:backend.src.services.notifications.service:Module]
 #
-# @TIER:      CRITICAL
+# @COMPLEXITY: 5
 # @SEMANTICS: notifications, service, routing, dispatch, background-tasks
 # @PURPOSE:   Orchestrates notification routing based on user preferences and policy context.
 # @LAYER:     Domain