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_tasks.py

@@ -72,7 +72,8 @@ class TestTasks:
             headers={'Authorization': f'Bearer {auth_token}'}
         )
 
-        assert response.status_code == 200
+        # DELETE should return 204 No Content (standard for successful deletion)
+        assert response.status_code == 204
 
     def test_user_isolation(self, client, db, test_user):
         """Test that users can only access their own tasks"""