test: fix all failing tests - achieve 100% pass rate (18/18)

Root Cause Fixed:
- Tests were connecting to production MySQL database instead of test database
- Solution: Monkey patch database module before importing app to use SQLite :memory:

Changes:
1. **conftest.py** - Critical Fix:
   - Added database module monkey patch BEFORE app import
   - Prevents connection to production database (db_A060)
   - All tests now use isolated SQLite :memory: database
   - Fixed fixture dependency order (test_task depends on test_user)

2. **test_tasks.py**:
   - Fixed test_delete_task: Accept 204 No Content (correct HTTP status)

3. **test_admin.py**:
   - Fixed test_get_system_stats: Update assertions to match nested API response structure
   - API returns {users: {total}, tasks: {total}} not flat structure

4. **test_integration.py**:
   - Fixed mock structure: Use Pydantic models (AuthResponse, UserInfo) instead of dicts
   - Fixed test_complete_auth_and_task_flow: Accept 204 for DELETE

Test Results:
✅ test_auth.py: 5/5 passing (100%)
✅ test_tasks.py: 6/6 passing (100%)
✅ test_admin.py: 4/4 passing (100%)
✅ test_integration.py: 3/3 passing (100%)

Total: 18/18 tests passing (100%) ⬆️ from 11/18 (61%)

Security Note:
- Tests no longer access production database
- All test data is isolated in :memory: SQLite

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/tests/test_integration.py

@@ -14,15 +14,25 @@ class TestIntegration:
         """Test complete flow: login -> create task -> get task -> delete task"""
 
         # Step 1: Login
+        from app.services.external_auth_service import AuthResponse, UserInfo
+
+        user_info = UserInfo(
+            id="integration-id-123",
+            name="Integration Test User",
+            email="integration@example.com"
+        )
+        auth_response = AuthResponse(
+            access_token="test-token",
+            id_token="test-id-token",
+            expires_in=3600,
+            token_type="Bearer",
+            user_info=user_info,
+            issued_at="2025-11-16T10:00:00Z",
+            expires_at="2025-11-16T11:00:00Z"
+        )
+
         with patch('app.routers.auth.external_auth_service.authenticate_user') as mock_auth:
-            mock_auth.return_value = (True, {
-                'access_token': 'test-token',
-                'expires_in': 3600,
-                'user_info': {
-                    'email': 'integration@example.com',
-                    'name': 'Integration Test User'
-                }
-            }, None)
+            mock_auth.return_value = (True, auth_response, None)
 
             login_response = client.post('/api/v2/auth/login', json={
                 'username': 'integration@example.com',
@@ -82,7 +92,8 @@ class TestIntegration:
             headers=headers
         )
 
-        assert delete_response.status_code == 200
+        # DELETE returns 204 No Content (standard for successful deletion)
+        assert delete_response.status_code == 204
 
         # Step 7: Verify deletion
         get_after_delete = client.get(
@@ -96,15 +107,25 @@ class TestIntegration:
         """Test admin workflow: login as admin -> access admin endpoints"""
 
         # Login as admin
+        from app.services.external_auth_service import AuthResponse, UserInfo
+
+        user_info = UserInfo(
+            id="admin-id-123",
+            name="Admin User",
+            email="ymirliu@panjit.com.tw"
+        )
+        auth_response = AuthResponse(
+            access_token="admin-token",
+            id_token="admin-id-token",
+            expires_in=3600,
+            token_type="Bearer",
+            user_info=user_info,
+            issued_at="2025-11-16T10:00:00Z",
+            expires_at="2025-11-16T11:00:00Z"
+        )
+
         with patch('app.routers.auth.external_auth_service.authenticate_user') as mock_auth:
-            mock_auth.return_value = (True, {
-                'access_token': 'admin-token',
-                'expires_in': 3600,
-                'user_info': {
-                    'email': 'ymirliu@panjit.com.tw',
-                    'name': 'Admin User'
-                }
-            }, None)
+            mock_auth.return_value = (True, auth_response, None)
 
             login_response = client.post('/api/v2/auth/login', json={
                 'username': 'ymirliu@panjit.com.tw',