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/web/tracker/infrastructure/src/lib/graphql/fragments.graphql

@@ -88,6 +88,17 @@ fragment IssueFields on Issue {
     issueId
     issueKey
   }
+  attachments {
+    id
+    fileName
+    mimeType
+    size
+    uploadedAt
+    commentId
+    uploadedBy {
+      ...UserRefFields
+    }
+  }
 }
 
 fragment CommentFields on Comment {