Tasks API: Implement GET /tasks and GET /tasks/{id} #899

Implement list/show endpoints.

Base path: /api/v2/organizations/{org}/members/{user}

Behavior

• List filters: status, template_ids, recipe_ids, ids.
• Optional: pagination.
• Show: return task details including status.
• Enforce RBAC and org scoping.
• No server-side bulk operations, client fan-out OK.

Response model (task)

• id: uuid
• organization_id: uuid
• owner_id: uuid
• name: string (slug/workspace name)
• display_name: string
• template_id: uuid
• recipe_id?: uuid (post-MVP)
• workspace_id?: uuid
• status: queued | working | idle | paused | completed | failed
• created_at, updated_at, started_at?, completed_at?
• last_error?: string

References: #838