feat: add conditional bootstrap login + 23 admin login tests

- Add hash_password/verify_password to app/auth.py using bcrypt
- Fix /login to reject bootstrap creds (403) when admin users exist
- Add normal password-based login path for existing admin users
- Eagerly load User.roles to avoid lazy-load outside async session
- Fix test_user_models.py VARCHAR assertion (SQLAlchemy 2.0 uses 'string')
- Use shared-cache SQLite in conftest for endpoint-level tests
- Add 23 tests covering all 3 required scenarios plus edge cases:
  1. Bootstrap login with hardcoded creds when no admins → PASS (200)
  2. Bootstrap login with hardcoded creds when admins exist → FAIL (403)
  3. Password login for existing admin with hashed password → PASS (200)
- All 130 tests pass (107 existing + 23 new)

backend/tests/conftest.py

@@ -1,14 +1,31 @@
 """Shared test fixtures for kmtnflower backend tests."""
 
+import os
 import pytest
 from unittest.mock import patch
 
-# Override settings for tests — use SQLite so we don't need PostgreSQL
+# Set test environment vars BEFORE any app imports
+# This is module-level so it runs when pytest loads this conftest
+os.environ.setdefault("KMTN_DATABASE_URL", "sqlite+aiosqlite:///file::memory:?cache=shared")
+os.environ.setdefault("KMTN_ENV", "test")
+os.environ.setdefault("KMTN_JWT_SECRET_KEY", "test-secret-key-do-not-use")
+os.environ.setdefault("KMTN_ADMIN_USERNAME", "testadmin")
+os.environ.setdefault("KMTN_ADMIN_PASSWORD", "testpass123")
+os.environ.setdefault("KMTN_LOGS_DIR", "/tmp/kmtn_test_logs")
+os.environ.setdefault("KMTN_GEOLITE2_DB_PATH", "/nonexistent/GeoLite2-City.mmdb")
+os.environ.setdefault("KMTN_THEME_JSON_PATH", "/tmp/kmtn_test_theme.json")
+
+# Use shared-cache in-memory SQLite so all connections within the process
+# see the same in-memory database (critical for tests that call the
+# login endpoint which opens its own DB connections).
+TEST_DB_URL = "sqlite+aiosqlite:///file::memory:?cache=shared"
+
+
 @pytest.fixture(autouse=True)
 def override_settings():
     """Set test-only environment variables before any import of app modules."""
     env = {
-        "KMTN_DATABASE_URL": "sqlite+aiosqlite:///:memory:",
+        "KMTN_DATABASE_URL": TEST_DB_URL,
         "KMTN_ENV": "test",
         "KMTN_JWT_SECRET_KEY": "test-secret-key-do-not-use",
         "KMTN_ADMIN_USERNAME": "testadmin",
@@ -19,3 +36,40 @@ def override_settings():
     }
     with patch.dict("os.environ", env, clear=False):
         yield env
+
+
+@pytest.fixture(autouse=True)
+async def reconfigure_database():
+    """Reconfigure the SQLAlchemy engine to use shared in-memory SQLite.
+
+    Uses file::memory:?cache=shared so all connections within the process
+    see the same in-memory database (critical for tests that call the
+    login endpoint which opens its own DB connections).
+
+    Drops all tables at the START of each test to ensure isolation.
+    """
+    from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker
+
+    from app import database
+    from app.models import Base as ModelsBase
+
+    # Create a new SQLite async engine with shared cache
+    test_engine = create_async_engine(TEST_DB_URL, echo=False)
+    test_session = async_sessionmaker(
+        test_engine, class_=database.AsyncSession, expire_on_commit=False
+    )
+
+    # Replace the module-level engine and session factory
+    database.engine = test_engine
+    database.async_session = test_session
+
+    # Drop all tables to ensure test isolation (shared-cache persists across tests)
+    # Then recreate them so fixture-dependent tests have a schema to work with.
+    async with test_engine.begin() as conn:
+        await conn.run_sync(ModelsBase.metadata.drop_all)
+        await conn.run_sync(ModelsBase.metadata.create_all)
+
+    yield test_engine
+
+    # Dispose of the test engine after each test
+    await test_engine.dispose()