fix(comment): dismiss owner/admin authz + atomic conditional delete + 404-only onError (#329 review)

Maintainer escalation decision (B) + reviewer findings on the ephemeral-
suggestion PR.

Authz (decision B): POST /comments/dismiss-suggestion now gates the destructive
branch on owner-OR-space-admin, mirroring POST /comments/delete exactly (same
SpaceCaslAction.Manage / SpaceCaslSubject.Settings, same owner short-circuit,
same ForbiddenException). A non-owner non-admin who tries to dismiss another's
childless suggestion gets Forbidden before the service runs. Apply stays on
canEdit (accepting an edit is the editor's semantics), unchanged.

F1 [blocking] — atomic conditional delete closes the hasChildren→delete race.
New repo `deleteCommentIfChildless(id)` runs a single
`DELETE FROM comments WHERE id=:id AND NOT EXISTS (SELECT 1 FROM comments child
WHERE child.parent_comment_id = comments.id)` (verified by compiling the Kysely
expression to SQL — the correlated subquery references the OUTER comments.id).
deleteEphemeralSuggestion strips the mark first, then the conditional delete: if
it removed the row → commentDeleted + outcome 'deleted'; if a reply raced in
(0 rows) → fall back to resolveComment (outcome 'resolved') so the discussion and
the new reply survive. No reply can be cascade-deleted anymore.

F2 [warning] — the apply/dismiss onError success-noop is narrowed from 404||400
to 404 ONLY. A 400 means the comment is ALIVE (apply's 400 = the thread was
resolved-not-applied), so it now shows a real error (surfacing the server
message) and KEEPS the comment in cache instead of a false "applied" + dropping a
live thread.

F3 [suggestion] — the 404-race client tests assert the success toast fired.

Tests: server — dismiss authz (owner ok / non-owner-non-admin Forbidden /
space-admin ok), the delete→resolve race (hasChildren=false but conditional
delete returns 0 → resolve, no commentDeleted), delete-path asserts switched to
deleteCommentIfChildless; client — apply-400 and dismiss-400 (kept in cache, red,
not success) + the toast assertions.

server tsc clean, comment+collaboration jest green; client tsc clean, comment
vitest 54 passed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/core/comment/comment.controller.ts

@@ -258,13 +258,33 @@ export class CommentController {
 
     // Authorize BEFORE revealing any structural detail (metadata-disclosure
     // hygiene, mirroring apply-suggestion). Dismissing a suggestion does NOT
-    // change the page text — it only removes/resolves the comment — so require
-    // comment access (canComment), NOT edit access. A viewer allowed to comment
-    // but not edit can still dismiss a suggestion. The structural 400s
-    // (top-level / has-a-suggested-edit / not applied / not resolved) are
-    // re-checked by the service below.
+    // change the page text — it only removes/resolves the comment — so the
+    // page-level gate is comment access (canComment), NOT edit access. A viewer
+    // allowed to comment but not edit can still dismiss their own suggestion.
+    // The structural 400s (top-level / has-a-suggested-edit / not applied /
+    // not resolved) are re-checked by the service below.
     await this.pageAccessService.validateCanComment(page, user, workspace.id);
 
+    // AUTHZ (#338): a childless dismiss IRREVERSIBLY hard-deletes the comment,
+    // so — beyond canComment — restrict it to the comment owner OR a space
+    // admin, exactly like POST /comments/delete. canComment alone is not enough:
+    // it would let any bystander commenter erase another user's suggestion for
+    // good. (apply-suggestion deliberately stays on canEdit: accepting an edit
+    // is the editor's semantics, not the suggestion author's.)
+    const isOwner = comment.creatorId === user.id;
+    if (!isOwner) {
+      const ability = await this.spaceAbility.createForUser(
+        user,
+        comment.spaceId,
+      );
+      // Space admin can dismiss any suggestion.
+      if (ability.cannot(SpaceCaslAction.Manage, SpaceCaslSubject.Settings)) {
+        throw new ForbiddenException(
+          'You can only dismiss your own suggestions',
+        );
+      }
+    }
+
     return this.commentService.dismissSuggestion(comment, user, provenance);
   }