chore(security): drop passlib for direct bcrypt + reject CLI-flag check targets

- Replace passlib.CryptContext with bcrypt 4.2.1 directly (passlib is
  unmaintained and only emitted warnings when paired with bcrypt 4+).
  hash_password / verify_password now call bcrypt.checkpw / .hashpw, and
  verify_password is hardened against empty inputs without raising.
  scripts/hash_password.py + tests/conftest.py updated to match.
- status_checker.check_node() now rejects targets starting with '-' before
  any subprocess invocation, defending ping/tcp paths against arg-injection
  if an admin sets a check_target like '-O'.

Tests added:
- test_auth: expired JWT, malformed JWT, wrong-secret JWT, empty password
  vs empty server hash, verify_password edge cases.
- test_scan: _background_scan failure path marks ScanRun failed, leaves
  non-running terminal status alone, success path invokes run_scan.
- test_status_checker: ping/tcp invocations are bypassed when target or
  ip starts with '-'.

Backend coverage 89% → 90%.

backend/tests/test_status_checker.py

@@ -216,6 +216,31 @@ async def test_ping_uses_windows_args_on_win32():
     assert "-c" not in captured["args"]
 
 
+# --- check_node target validation ---
+
+@pytest.mark.asyncio
+async def test_check_node_rejects_flag_like_target():
+    """A target starting with '-' must never reach subprocess invocation."""
+    from app.services.status_checker import check_node
+
+    with patch("asyncio.create_subprocess_exec") as mock_exec:
+        result = await check_node("ping", "-O", None)
+
+    mock_exec.assert_not_called()
+    assert result["status"] == "unknown"
+
+
+@pytest.mark.asyncio
+async def test_check_node_rejects_flag_like_ip():
+    from app.services.status_checker import check_node
+
+    with patch("asyncio.create_subprocess_exec") as mock_exec:
+        result = await check_node("ping", None, "-O")
+
+    mock_exec.assert_not_called()
+    assert result["status"] == "unknown"
+
+
 # --- _tcp_connect ---
 
 @pytest.mark.asyncio