Add a new CSV migration module that allows users to import tasks from
any CSV file with custom column mapping and parsing options.
Backend changes:
- New CSV migrator module with detection, preview, and import endpoints
- Auto-detection of delimiter, quote character, and date format
- Suggested column mappings based on column name patterns
- Transactional import using InsertFromStructure
Frontend changes:
- New CSV migration UI with two-step flow (upload -> mapping -> import)
- Column mapping selectors for all task attributes
- Live preview showing first 5 tasks with current mapping
- Parsing option controls for delimiter and date format
The CSV migrator creates a parent "Imported from CSV" project with
child projects based on the project column if provided, or a default
"Tasks" project for tasks without a specified project.
