Tasks API: Implement POST /tasks #898

Implement create endpoint.

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

Behavior

• Body: { name?, template_id? | recipe_id?, parameters?: { [k]: string }, input?: string }
• Require one of template_id or recipe_id.
• Map input to prompt parameter (TBD constant).
• Generate name if absent.
• Return task details.
• MVP: template-based creation first, recipe-based creation post-MVP.

Request model

• name?: string
• template_id?: uuid
• recipe_id?: uuid (post-MVP)
• parameters?: map[string]string
• input?: string (shorthand for prompt)

Response model (task)

• id: uuid
• organization_id: uuid
• owner_id: uuid
• 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