[poisvin]

Summary

Implements Gather #7391 / T29: https://app.gatherai.co/tasks/7391

• Add optional Task contact/prospect references to existing sales records, with at most one reference set.
• Support create/read/list/replace/clear. Omitted PATCH fields preserve the reference; setting one person clears the other.
• Validate integer IDs, organization membership of the referenced record, and conflicting inputs. Reject nonexistent/cross-organization IDs without partial writes.
• Return minimal related id/name summaries; never infer or duplicate person identity.
• Filter repository writes to permitted task fields so nested Prisma operations cannot bypass person-reference validation.
• Preserve task assignment, ownership and existing permissions. No UI components or person-page buttons are added; web changes are API types only.

Migration

Add nullable contact_id/prospect_id, indexes, ON DELETE SET NULL foreign keys, and a check constraint preventing both links. No backfill. Deleting a person clears the reference without deleting the task.
Applied only to isolated local preview database zuko_cm_preview. Prisma migration diff reported no difference from the schema. No production deployment performed.

Verification

• Backend: 447 tests passed across 26 files.
• Sales library: 280 tests passed across 16 files, including real database reference tests.
• Frontend: 353 tests passed across 19 files.
• Web/backend/sales typechecks, formatting and whitespace checks passed.
• Live API checks passed for create/read/list, omission, replacement across reference types, clearing, invalid/conflicting/cross-org/nonexistent ID rejection, rollback, deletion safety and unchanged task ownership/assignment.
• Creating people does not create tasks. Temporary test fixtures removed.