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/conftest.py

@@ -5,23 +5,21 @@ os.environ.setdefault("SECRET_KEY", "test-only-secret-key-not-for-production")
 
 import pytest
 from httpx import ASGITransport, AsyncClient
-from passlib.context import CryptContext
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
 
+from app.core.security import hash_password
 from app.db.database import Base, get_db
 from app.main import app
 
 TEST_DB_URL = "sqlite+aiosqlite:///:memory:"
 
-_pwd_ctx = CryptContext(schemes=["bcrypt"], deprecated="auto")
-
 
 @pytest.fixture(autouse=True, scope="session")
 def test_credentials():
     """Configure test auth credentials directly on settings."""
     from app.core.config import settings
     settings.auth_username = "admin"
-    settings.auth_password_hash = _pwd_ctx.hash("admin")
+    settings.auth_password_hash = hash_password("admin")
 
 
 @pytest.fixture