API: Support created_at for API card and comment creation (#2056)

* Add support to set `created_at`

Discussed in
https://github.com/basecamp/fizzy/pull/1766#issuecomment-3637846074,
this allows users to import cards from another system with entries from
the past. For instance I'm importing all our Github issues (including
the closed onces) to Fizzy.

* Iron out published state tracking

---------

Co-authored-by: Jeremy Daer <jeremy@37signals.com>

docs/API.md

@@ -494,6 +494,7 @@ Creates a new card in a board.
 | `status` | string | No | Initial status: `published` (default), `drafted` |
 | `image` | file | No | Header image for the card |
 | `tag_ids` | array | No | Array of tag IDs to apply to the card |
+| `created_at` | datetime | No | Override creation timestamp (ISO 8601 format) |
 
 __Request:__
 
@@ -699,6 +700,7 @@ Creates a new comment on a card.
 | Parameter | Type | Required | Description |
 |-----------|------|----------|-------------|
 | `body` | string | Yes | The comment body (supports rich text) |
+| `created_at` | datetime | No | Override creation timestamp (ISO 8601 format) |
 
 __Request:__