Add CSV import for life events

[26tajeen]

Summary

• add a CSV import step to the integration options flow
• validate and normalize imported events, with merge/replace modes
• restrict imports to the Home Assistant config directory and 1 MB
• document the CSV format and add unit coverage

CSV format

Required columns: name, date, and type. Optional columns: year_unknown, custom_label, and icon. Duplicate detection uses normalized name, date, and event type.

Validation

• flake8 custom_components/life_events/ tests/ --max-line-length=120 --ignore=E501,W503
• JSON syntax checks for strings and translations
• git diff --check

This is intentionally separate from the bulk-management/card changes so it can be reviewed independently.