feat(assignment): classifier proposes assignee, human overrides

Extend the classifier to suggest an assignee based on task metadata; human accepts or overrides.

## Scope
- New `Task.suggestedAssigneeId` column (nullable, separate from `assigneeId`).
- Extend `ClassifierConfig` output shape to include a proposed assignee.
- Classifier reads: labels, projectId, keyword patterns in title/body, past resolver history (AuditLog joined to Task.assigneeId).
- Inline control shows suggestion with a one-click "accept" affordance — no auto-apply without human confirmation.
- If OPER-2 rule matches, that wins (rules are stronger than suggestions).

## Depends on
- OPER-1 (UI seam)
- OPER-2 (rules take precedence)

## Verification
- 10 test tickets classified via `POST /api/studio/{team}/tasks` (BACKLOG intake).
- `suggestedAssigneeId` populated on each.
- Sensible suggestions on at least 7/10 (spot-check).