Projects can span multiple counterparties

Projects are now treated as free-form buckets - counterparty membership
is derived from the documents in the project (intake_counterparty),
plus an optional projects.counterparty as a "primary" hint. A project
named "Customer Contracts" with Adobe + Stripe + Intercom docs now
appears under each of those three counterparties in /customers.

- GET /projects/counterparties: project rows pull every distinct
  intake_counterparty across the project's docs; project_count is
  incremented per-counterparty. Role filter applies at the document
  level so a multi-cp project surfaces correctly.
- GET /projects/counterparties/:name/timeline: includes any project
  whose docs reference this counterparty (not just primary-counterparty
  matches). Document list is scoped to docs that actually belong to
  this counterparty (or all docs when this IS the project's primary).