API: Allow updates to last_active_at (#2076)

* Allow Card#last_updated_at to be set

This is useful when doing an import from another system. I'm currently
working on a script to import our Github issues into Fizzy.

This is discussed in
https://github.com/basecamp/fizzy/pull/2056#discussion_r2609560246

* Add nil fallback and expand test coverage for last_active_at

Adds a safety fallback to Time.current if created_at is unexpectedly nil
during card creation.

Test coverage to verify:
* last_active_at defaults to created_at when not provided
* last_active_at can be updated via API on existing cards
* import workflow where last_active_at is restored after comments
* publishing doesn't overwrite explicit last_active_at values

---------

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

docs/API.md

@@ -496,6 +496,7 @@ Creates a new card in a board.
 | `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) |
+| `last_active_at` | datetime | No | Override last activity timestamp (ISO 8601 format) |
 
 __Request:__
 
@@ -523,6 +524,7 @@ Updates a card.
 | `status` | string | No | Card status: `drafted`, `published` |
 | `image` | file | No | Header image for the card |
 | `tag_ids` | array | No | Array of tag IDs to apply to the card |
+| `last_active_at` | datetime | No | Override last activity timestamp (ISO 8601 format) |
 
 __Request:__