test: add pytest markers (unit/integration/database) and fix categorization

[dtsong]

Problem

make test-unit hardcodes 2 test files. No markers distinguish unit from integration tests. New unit test files are not automatically included.

Scope

1. Add custom pytest markers: unit, integration, database, slow
2. Mark existing tests appropriately
3. Update Makefile: make test-unit → uv run pytest -m unit
4. Register markers in pyproject.toml or conftest.py

Acceptance Criteria

• Markers registered and documented
• Existing tests marked
• make test-unit uses marker-based selection