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

README.md

@@ -155,16 +155,47 @@ To backup your data:
 cp tablets.db tablets_backup_$(date +%Y%m%d).db
 ```
 
-## Version Control
+## Testing
 
-This project uses Git for version control with the following structure:
+The project includes a comprehensive unit test suite with **46 tests** covering core functionality and edge cases.
+
+### Running Tests
+
+```bash
+# Install test dependencies (if not already installed)
+source .venv/bin/activate
+uv pip install pytest pytest-cov
+
+# Run all tests
+python3 -m pytest tests/ -v
+
+# Run with coverage report
+python3 -m pytest tests/ --cov=./ --cov-report=term-missing
+```
+
+### Test Structure
+
+| File | Tests | Coverage |
+|------|-------|----------|
+| `tests/test_core.py` | 21 | Core operations (tablet, user, loan CRUD) |
+| `tests/test_edge_cases.py` | 14 | Edge cases (duplicates, invalid IDs, etc.) |
+| `tests/test_minimal_app.py` | 11 | Application function tests |
+
+### Key Edge Cases Covered
+
+- Loan a device that's already loaned (prevents double-loaning)
+- Return a non-existent loan (handles gracefully)
+- Duplicate serial numbers (rejected at database level)
+- Duplicate user identifications (rejected at database level)
+- Invalid tablet/user IDs (validated before operations)
+- Multiple loans per user (one-to-many relationship)
+- Loan-return-loan sequence (device lifecycle)
+
+### Test Configuration
+
+Tests use isolated temporary databases and automatically clean up after each test run. No impact on production data.
 
-- `master` - Production-ready releases
-- `develop` - Integration branch for features
-- `feature/*` - Individual feature branches
-- `hotfix/*` - Urgent bug fixes
 
-See `CONTRIBUTING.md` for detailed workflow and commit conventions.
 
 ## License