[LA-36814] Doc status filter + item channels endpoints

[tejanium]

Jira

https://learnamp.atlassian.net/browse/LA-36814

What

Documents two parts of the public API that already exist in code but were missing from the docs:

• GET /v1/items/:id/channels — new "List Channels Containing an Item" section in _items.md, mirroring the existing "List Learnlists Containing an Item". Returns the channels (Carousels) containing an item; response is { "channels": [{ "id", "title" }] }, paginated, items:read scope.
• filters[status] on GET /v1/users — new row in the View All Users filter table in _users.md. Multi-value; possible values: not_invited, invite_scheduled, invite_pending, confirmed, deactivated.

Why

Part of LA-36515 (Filter parity on list APIs). Customers (Namecheap, Winc Academy) need these documented to use them. The /items/:id/channels endpoint (LA-36812) and the status filter (LA-36813) are delivered in sibling learnamp PRs #23667 and #23668. filters[no_team] and /items/:id/learnlists were already documented, so they're untouched here.

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Note

Low Risk
Markdown-only API documentation; no runtime or security behavior changes.

Overview
Documents two public API capabilities that already ship in the product but were missing from the docs.

Items: Adds a List Channels Containing an Item section for GET /v1/items/{itemId}/channels, aligned with the existing learnlists section—curl/Ruby examples, items:read scope, pagination, and a { "channels": [{ "id", "title" }] } response plus 404.

Users: Adds filters[status][] to the View All Users filter table so callers can filter by one or more statuses (not_invited, invite_scheduled, invite_pending, confirmed, deactivated) via repeated query params.

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