fix: resolve 57 ruff lint errors (unused imports, unused variables, import placement)

tests/test_api_main.py

@@ -1,6 +1,5 @@
 # tests/test_api_main.py — Integration tests for main.py API endpoints
 # Uses the existing app_with_vault / client fixtures from conftest.py
-import pytest
 
 
 # ═══════════════════════════════════════════════════════════════════
@@ -416,7 +415,7 @@ class TestConfig:
             "search_workers": 4,
         })
         assert resp.status_code == 200
-        data = resp.json()
+        _ = resp.json()
         # POST /api/config returns the updated config object directly
 
     def test_diagnostics(self, client):
@@ -622,8 +621,6 @@ class TestBackupFile:
     def test_backup(self, tmp_path, monkeypatch):
         """Backup is created by save operations"""
         from backend.main import _backup_file
-        from pathlib import Path
-        import os
         monkeypatch.chdir(tmp_path)
         vault = tmp_path / "Vault"
         vault.mkdir()