feat: filename template expansion with {date} placeholder

- Add expand_filename_template() to yaml_config.py
- Supports {author}, {from}, {to}, {date}, {ext} placeholders
- {date} formats as DD_MM_YYYY (e.g. 31_03_2026)
- Spaces in author replaced with underscores for safe filenames
- Unknown placeholders left as-is
- Add comprehensive CONFIG.md documentation covering YAML setup, migration, and template syntax

tests/test_config.py

@@ -5,7 +5,7 @@ from unittest import mock
 
 import pytest
 
-from redmine_reporter.config import AppConfig, Config, DEFAULT_REDMINE_VERIFY
+from redmine_reporter.config import DEFAULT_REDMINE_VERIFY, AppConfig, Config
 
 
 @mock.patch.dict(