fix(backup): address Codacy findings

- api_v3: replace 'fonts' in ' '.join(result.restored) substring check
  with any(r.startswith("fonts") for r in result.restored) to avoid
  fragile joined-string membership testing
- api_v3: replace deprecated datetime.utcnow() and utcfromtimestamp()
  with datetime.now(timezone.utc) and fromtimestamp(..., timezone.utc);
  add timezone to import
- test: remove unused import io (backup_manager no longer uses BytesIO)
- src/backup_manager.py hardcoded /tmp sentinel was already fixed in a
  prior commit (tempfile.TemporaryDirectory)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/test_backup_manager.py

@@ -2,7 +2,6 @@
 
 from __future__ import annotations
 
-import io
 import json
 import zipfile
 from pathlib import Path

web_interface/blueprints/api_v3.py

@@ -8,7 +8,7 @@ import time
 import hashlib
 import uuid
 import logging
-from datetime import datetime
+from datetime import datetime, timezone
 from pathlib import Path
 from typing import Optional, Tuple, Dict, Any, Type
 
@@ -1147,7 +1147,7 @@ def backup_export():
             'status': 'success',
             'filename': zip_path.name,
             'size': zip_path.stat().st_size,
-            'created_at': datetime.utcnow().isoformat() + 'Z',
+            'created_at': datetime.now(timezone.utc).isoformat(),
         })
     except Exception:
         logger.exception("[Backup] export failed")
@@ -1167,7 +1167,7 @@ def backup_list():
             entries.append({
                 'filename': path.name,
                 'size': stat.st_size,
-                'created_at': datetime.utcfromtimestamp(stat.st_mtime).isoformat() + 'Z',
+                'created_at': datetime.fromtimestamp(stat.st_mtime, timezone.utc).isoformat(),
             })
         return jsonify({'status': 'success', 'data': entries})
     except Exception:
@@ -1355,7 +1355,7 @@ def backup_restore():
                     result.plugins_failed.append({'plugin_id': plugin_id, 'error': str(install_err)})
 
         # Clear font catalog cache so restored fonts show up.
-        if 'fonts' in ' '.join(result.restored):
+        if any(r.startswith("fonts") for r in result.restored):
             try:
                 from web_interface.cache import delete_cached
                 delete_cached('fonts_catalog')