chore(lint): apply ruff --fix (4443 auto-fixes)

Auto-fixed categories:
- F401: unused imports removed
- I001: import blocks sorted
- W293: trailing whitespace stripped
- UP035: deprecated typing imports replaced
- SIM: simplify suggestions applied
- ARG: unused args prefixed with underscore
- T201: print statements removed
- F841: unused variables removed
- RUF059: unpacked variables prefixed

Remaining ~1500 unfixable errors (C901, B904, N806, E402) require manual work.

Backend smoke tests: 13/13 passed.

backend/tests/test_dashboards_api.py

@@ -3,24 +3,26 @@
 # @PURPOSE: Comprehensive contract-driven tests for Dashboard Hub API
 # @LAYER: Domain (Tests)
 # @SEMANTICS: tests, dashboards, api, contract, remediation
+from datetime import UTC, datetime
+from unittest.mock import AsyncMock, MagicMock, patch
+
 import pytest
 from fastapi.testclient import TestClient
-from unittest.mock import MagicMock, patch, AsyncMock
-from datetime import datetime, timezone
-from src.app import app
+
 from src.api.routes.dashboards import (
-    DashboardsResponse,
     DashboardDetailResponse,
+    DashboardsResponse,
     DashboardTaskHistoryResponse,
     DatabaseMappingsResponse,
 )
+from src.app import app
 from src.dependencies import (
-    get_current_user,
-    has_permission,
     get_config_manager,
-    get_task_manager,
-    get_resource_service,
+    get_current_user,
     get_mapping_service,
+    get_resource_service,
+    get_task_manager,
+    has_permission,
 )
 
 # Global mock user
@@ -299,7 +301,7 @@ def test_get_dashboard_detail_env_not_found(mock_deps):
 # [DEF:test_get_dashboard_tasks_history_success:Function]
 # @RELATION: BINDS_TO ->[TestDashboardsApi]
 def test_get_dashboard_tasks_history_success(mock_deps):
-    now = datetime.now(timezone.utc)
+    now = datetime.now(UTC)
     task1 = MagicMock(
         id="t1",
         plugin_id="superset-backup",
@@ -328,7 +330,7 @@ def test_get_dashboard_tasks_history_sorting(mock_deps):
     """@POST: Response contains sorted task history (newest first)."""
     from datetime import timedelta
 
-    now = datetime.now(timezone.utc)
+    now = datetime.now(UTC)
     older = now - timedelta(hours=2)
     newest = now