Open a TaskStream board when a project is created through the API

`taskstream: true` on POST /v1/projects also creates the project in
TaskStream, public, as the caller (their Pireon token is passed on), and
the answer carries `taskstream_key` and `taskstream_url`. The key is
`taskstream_key` or one made from the default-language slug.

The board is opened before the project is saved: if TaskStream refuses
it, nothing is created here and the reason comes back as a validation
error (502 when TaskStream does not answer). Machine tokens need the
server's own TASKSTREAM_TOKEN. Updates ignore the flag, and duplicates do
not inherit the link.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lang/en/rules.php

@@ -48,6 +48,14 @@ return [
         'event' => '":event" is not an event that can be subscribed to.',
     ],
 
+    'taskstream' => [
+        'machine' => 'TaskStream only understands Pireon tokens: sign in with one, or set TASKSTREAM_TOKEN for machine tokens.',
+        'key_needed' => 'No key can be made from the slug: give one in "taskstream_key" (2 to 10 letters or digits, starting with a letter).',
+        'refused' => 'TaskStream did not create the project: :reason',
+        'unauthorised' => 'TaskStream rejected the token: it may have expired or lack access.',
+        'unavailable' => 'TaskStream is not answering right now; nothing was created, try again in a while.',
+    ],
+
     'tokens' => [
         'ability' => '":ability" is not a permission you hold, so a token of yours cannot hold it.',
         'machine' => 'Tokens are managed by a person, not by another token.',