refactor: populate conftest.py with shared fixtures and auto-skip logic

[dtsong]

Problem

conftest.py is empty (1 line). All shared test infrastructure lives in common.py using unittest patterns. The CONN_STRINGS dict in common.py:103 silently filters out databases without connection strings — tests pass with zero coverage and no skip messages.

Scope

1. Add session-scoped database connection fixtures to conftest.py
2. Add custom markers: pytest.mark.database("postgresql") etc.
3. Replace silent filter with explicit pytest.mark.skip(reason="...")
4. New tests should follow pytest-native style (not unittest.TestCase)

Acceptance Criteria

• conftest.py provides shared fixtures
• Skipped databases show as SKIPPED (not silently filtered)
• pytest output clearly shows which databases were tested vs skipped