OpenClear 1.13.0

Reporting through each bank's own business intelligence platform: a guide, and what a data
pipeline needs from the API. It brings:

• docs/guides/bi.md: connecting Power BI, Tableau, Looker, Qlik or Superset through a warehouse;
• updated_since on every list, with cursor paging that never misses a row;
• read-only API clients, including a customer's own data feed;
• the full data export through the API;
• for the sandbox, the caller's real address behind Fly's proxy.

For banks' data pipelines and BI platforms (docs/guides/bi.md)

• updated_since on every list (exceptions, issued and presented checks, stop payments,
  ACH filters and transactions, accounts, customers): only rows changed since a time, in the
  order they changed. Pages follow a Link: rel="next" cursor (updated_since with
  after_id), so a row that changes mid-way is never skipped. Each row now carries
  updated_at and customer_id.
• Read-only API clients: tick "Read only" when creating a client; whatever its group allows,
  it can only read (and run the full data export). Limited to one customer, it's that
  customer's own data feed.
• The full data export through the API: POST /api/v1/data-exports (the bank, or one
  customer with customer_number; a customer's client always its own), poll
  GET /api/v1/data-exports/{id}, download file_url.
• The BI guide covers all three.
• Migration a4c6e8a0c2d5: updated_at on exceptions, presented checks, ACH transactions
  and filters, stop payments, accounts and customers (from their creation time), indexes for
  changed-since reads, and api_client.read_only.