docs: document party role endpoints and the party assignment flow

[maurycy]

What

• openapi.json: GET/POST /{shipments,containers}/{id}/party_roles, DELETE .../party_roles/{id}, the party_role schema, and flag[parties] on GET /shipments and GET /shipments/{id}. Party descriptions no longer say "customer only". The tracking request customer relationship in responses is documented as an account or null, which is what the serializer returns.
• New reference group Party Roles (6 pages) next to Parties.
• New guide Assign Shipper, Consignee, and Other Parties: create party, get the shipment ID from the tracking request, assign roles on the shipment, read them back with flag[parties]=true&include=party_roles.party, replace a role, container dray carrier, error table.
• adding-customer guide: cross-link, corrected response example (customer is null for parties without a linked account).
• Changelog entry for September 2, 2026. Older entry reworded from "unbacked party" to "party without a linked account".

Tracking request party role endpoints exist but are left undocumented on purpose.

Why

A customer asked for an API way to set shipper, consignee, and freight forwarder instead of the dashboard bulk update. The endpoints exist and accept API keys, but were undocumented.

Verification

• Every endpoint, status code, and error message in the docs was exercised with Authorization: Token against tnt-api request specs (not committed).
• mint openapi-check: valid. mint broken-links: none. git diff --check: clean.
• Postman collection is regenerated by CI.

🤖 Generated with Claude Code

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