Adds a DB API 2.0 (PEP 249) compatible module that wraps LogfireQueryClient,
enabling Logfire query data to work with any tool that supports standard Python
database connections — pandas read_sql(), marimo SQL cells, Jupyter %%sql, etc.

…ions

- Add default min_timestamp of 30 days ago (accepts datetime, timedelta, or None)
- Reject str/bytes as parameters in _substitute_params
- Use pytest.warns for truncation warning test
- Fix skip directive in marimo docs example

- Use `if parameters is not None:` so empty dict/list still triggers
  `%%` → `%` conversion per pyformat paramstyle
- Remove unused `marimo` import from docs code block

…on-level filters

Previously, cursor-level min_timestamp/max_timestamp used `or` fallback
which made it impossible to explicitly disable a connection-level timestamp
filter from a cursor (None would always fall through to the connection value).
Now _UNSET means "inherit from connection" and None means "no filter".

When limit=None was passed through, the server would silently cap at 500
rows and our truncation warning wouldn't fire since effective_limit was
None. Now cursor.limit is typed as int, always resolving to a concrete
value.