fix(upload): link attachments to comments via client-side ID tracking

Instead of regex-parsing markdown content to find attachment URLs
(fragile), the frontend now tracks uploaded attachment IDs and sends
them with the comment creation request. The backend links them by ID.

Frontend: upload returns attachment ID, comment/reply inputs collect
IDs during editing session, pass as attachment_ids on submit.
Backend: CreateComment accepts attachment_ids, links by ID+issue scope.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/pkg/db/queries/attachment.sql

@@ -31,5 +31,12 @@ WHERE a.issue_id = $1
 SELECT url FROM attachment
 WHERE comment_id = $1;
 
+-- name: LinkAttachmentsToComment :exec
+UPDATE attachment
+SET comment_id = $1
+WHERE issue_id = $2
+  AND comment_id IS NULL
+  AND id = ANY($3::uuid[]);
+
 -- name: DeleteAttachment :exec
 DELETE FROM attachment WHERE id = $1 AND workspace_id = $2;