fix: restore .worktrees/ entry in .gitignore and add backend/tests/__init__.py

- Restored .worktrees/ line that was lost when .gitignore was overwritten in commit 1736dd0
- Added backend/tests/__init__.py as a package marker (was untracked on disk)
- Test suite verified: backend tests pass (1/1)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -1,3 +1,4 @@
+.worktrees/
 backend/venv/
 __pycache__/
 *.pyc