feat(file_discovery): add discover_year_range and info logging (#428)

* feat(file_discovery): add discover_year_range and info logging

- Add discover_year_range() public function that returns (first_year, last_year)
  from filenames only — no file I/O.
- discover_files() now logs an info message with the file count and year range
  on every call.

* docs(notebook): add discover_year_range example to Getting_started

* Select time range in the notebook example

* pre-commit

* test(file_discovery): add tests for discover_year_range and logging

* refactor(test_file_discovery): reorder logging import for consistency

Author: rbeucher