test: add comprehensive unit test suite

- Add tests/ directory with pytest configuration
- Add conftest.py with database fixtures (test_db, sample data)
- Add test_core.py: 21 tests for core operations (tablet, user, loan CRUD)
- Add test_edge_cases.py: 14 tests for edge cases (duplicates, invalid IDs, etc.)
- Add test_minimal_app.py: 11 tests for actual application functions
- Update pyproject.toml with pytest and coverage configuration
- Total: 46 tests covering core functionality and edge cases

Tests cover:
- Tablet CRUD operations
- User CRUD operations
- Loan/return workflows
- Duplicate prevention (serial numbers, identifications)
- Invalid ID handling
- Already loaned device prevention
- Non-existent loan handling
- Multiple loans per user (one-to-many relationship)
- Non-loanable device CRUD operations

To run: python3 -m pytest tests/ -v

pyproject.toml

@@ -5,3 +5,33 @@ description = "Add your description here"
 readme = "README.md"
 requires-python = ">=3.14"
 dependencies = []
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+python_files = "test_*.py"
+python_classes = "Test*"
+python_functions = "test_*"
+verbose = 1
+addopts = "-v"
+
+[tool.coverage.run]
+source = ["."]
+omit = [
+    "*/tests/*",
+    "*/.venv/*",
+    "*/__pycache__/*",
+    "*/.git/*",
+    "*/templates/*",
+]
+
+[tool.coverage.report]
+exclude_lines = [
+    "pragma: no cover",
+    "def __repr__",
+    "raise NotImplementedError",
+    "if __name__ == .__main__.:",
+    "if TYPE_CHECKING:",
+]
+
+[tool.coverage.html]
+directory = "htmlcov"