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/domain/src/index.ts

@@ -2,6 +2,7 @@ export * from './lib/project/project';
 export * from './lib/project/project-role';
 export * from './lib/issue/issue';
 export * from './lib/issue/issue.enums';
+export * from './lib/issue/issue-attachment';
 export * from './lib/project/project-settings';
 export * from './lib/comment/comment';
 export * from './lib/release/release';