[P5] Add 'savings' as valid item_type in line_items

Summary

Add savings as a valid item_type in the line_items table. Currently only income and expense are accepted.

Requirements

1. Update the line_items.item_type column in db/schema.sql to document savings as a valid value (remove or update inline comment)
2. Update migrate_db() in server/db.py to handle existing DBs — no schema ALTER needed since there's no CHECK constraint, but ensure seeds include savings items
3. Seed a 'Investments & Savings' line item (item_type='savings') in the Personal ledger default seeds in server/db.py init_db()
4. Update add_line_item MCP tool to accept 'savings' as valid item_type (currently validates income | expense only — update that validation)
5. Update any validation in ui/server.py that restricts item_type
6. Update ARCHITECTURE.md line_items section to note savings as a valid type
7. Add tests in tests/ covering the new item_type

Why

Foundation for savings tracking. Issues #235, #248, and #237 all depend on this.

Acceptance criteria

• add_line_item accepts item_type='savings' without error
• Default Personal ledger seeds include an 'Investments & Savings' savings line item
• All existing tests pass

Depends on

Nothing — this is the foundation.