Add fetch command for per-account JSON export

[robcohen]

Summary

Adds a new simplefin fetch command that fetches all accounts with their transactions and saves each account to a separate JSON file.

Features

• Fetches all accounts in a single session, then retrieves transactions for each
• Outputs one JSON file per account (useful for tools like beangulp that expect one file per account)
• Organizes files hierarchically: <institution>/<account-name>/<account-id>_<date>.json
• Preserves history with timestamped filenames
• Handles duplicate account names correctly (same folder, unique filenames)

Usage

simplefin fetch --output-dir ./simplefin-data --lookback-days 30

Creates:

simplefin-data/
  www.chase.com/
    Chase-Checking/
      ACT-abc123_2025-01-15.json
  www.bankofamerica.com/
    Savings/
      ACT-def456_2025-01-15.json

Use Case

This is particularly useful for integration with beangulp importers, which expect one file per account. The hierarchical structure is human-navigable while the account-id in filenames ensures uniqueness.

Changes

• src/simplefin/cli/__init__.py: Add fetch command
• tests/test_cli.py: Add 6 tests for the new command
• README.md: Add documentation for the new command

Test Plan

• All 6 new tests pass
• Tested manually with real SimpleFIN account (44 accounts)
• Verified directory structure and JSON content