[poisvin]

Summary

Implements Gather #7390: https://app.gatherai.co/tasks/7390

• Add nullable Task.assigneeUserId and related user summary. Preserve creator/owner links: TaskOwner records ownership, not assignment.
• Resolve names server-side and validate that a selected user belongs to the organization. Write ID/name atomically. Omission preserves assignment; null clears it.
• Keep existing legacy name-only records without inferred matches or a backfill. Legacy API/MCP/table name writes clear any conflicting stable ID.
• Use user IDs in TaskForm, TaskDetail and TasksList selectors, with email labels to distinguish duplicate names. Display current related names after renames.
• Preserve task permissions and reuse existing UI components.

Migration

Adds nullable assignee_user_id, index and foreign key with ON DELETE SET NULL. No data backfill. Applied and tested only on the isolated local preview database; production migration/deployment is not performed by this PR creation.

Verification

• Frontend: 353 tests passed across 19 files.
• Backend: 381 tests passed across 26 files.
• Sales library: 252 tests passed across 15 files, including real database assignment tests.
• Web, backend and sales typechecks passed; formatting and whitespace checks passed.
• Live API checks passed for create/reassign/clear, omitted updates, duplicate names, rename stability, invalid/foreign IDs, legacy compatibility and unchanged ownership.
• Visible-browser checks passed for form assignment/edit plus Tasks table reassignment and clearing. Temporary local test records removed.

Test output includes existing task-priority query warnings and pg deprecation warnings; all suites above passed on final execution. No independent reviewer approval claimed.