docs: add testing documentation to README and RUNNING.md

- Add Testing section to README with:
  - How to run tests
  - Test structure overview
  - Key edge cases covered
  - Test configuration notes
- Add Running Tests section to RUNNING.md
- Document the 46 unit tests available

RUNNING.md

@@ -48,10 +48,22 @@ pkill -f "python app.py"
 - **Port**: 5000 (configurable in app.py)
 - **Virtual Environment**: `.venv/` (created with uv)
 
-## Troubleshooting
-If you have issues:
-1. Check if the server is running: `ps aux | grep app.py`
-2. Check the database: `sqlite3 tablets.db`
-3. Restart the server: `source .venv/bin/activate && python app.py`
+## Running Tests
 
-Enjoy managing your tablets! 📱💻
+The project includes 46 unit tests for verifying functionality:
+
+```bash
+# Run all tests
+python3 -m pytest tests/ -v
+
+# Run with coverage
+python3 -m pytest tests/ --cov=./ --cov-report=term-missing
+```
+
+Tests cover:
+- Core CRUD operations for tablets, users, and loans
+- Edge cases (duplicates, invalid IDs, already loaned devices)
+- Application function testing
+- Non-loanable device management
+
+All tests use isolated temporary databases and do not affect production data.