feat: attachments on tickets and comments

Files are uploaded over REST, stored on disk (a volume in Docker) and
recorded on the ticket as events; the bytes are written first, so no
attachment ever points at a missing file. Only images, video, audio, PDF
and plain text are shown inline, everything with nosniff and a sandbox CSP.
The web shows thumbnails, uploads by drag and drop or by pasting a
screenshot, and attaches files to new comments.

Also adds `nx run api:contracts`, which writes schema.gql and
openapi.json without starting the API or any database.

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

libs/api/tracker/application/src/lib/event-handlers/activity.event-handler.ts

@@ -5,6 +5,8 @@ import {
   CommentDeletedEvent,
   CommentEditedEvent,
   IssueAssignedEvent,
+  IssueAttachmentAddedEvent,
+  IssueAttachmentRemovedEvent,
   IssueCreatedEvent,
   IssueLinkedEvent,
   IssueReleaseChangedEvent,
@@ -51,6 +53,8 @@ import { versionOf } from './read-model-notifier';
   IssueLinkedEvent,
   IssueUnlinkedEvent,
   IssueReleaseChangedEvent,
+  IssueAttachmentAddedEvent,
+  IssueAttachmentRemovedEvent,
   CommentAddedEvent,
   CommentEditedEvent,
   CommentDeletedEvent,