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/commands/detach-file.command.ts

@@ -0,0 +1,12 @@
+import { Command } from '@nestjs/cqrs';
+import { CommandResult } from './command-result';
+
+export class DetachFileCommand extends Command<CommandResult> {
+  constructor(
+    public readonly actorId: string,
+    public readonly issueId: string,
+    public readonly attachmentId: string,
+  ) {
+    super();
+  }
+}