feat(suggestions): visible, reversible rejection in the modal rail

A red-✗ dismissal no longer makes the suggestion vanish. The rejected tag
stays in the rail — dimmed, struck-through, with a "rejected" pill and a
one-click undo (↶) in place of the ✗ — so a misclick is recoverable and the
operator can see what they've said no to (operator-asked 2026-06-27).

Backend: SuggestionService.for_image now KEEPS rejected tags, flagged
rejected=True, sorted to the bottom of their category, instead of dropping
them. New AllowlistService.undismiss + POST /suggestions/undismiss clears the
TagSuggestionRejection. Rejected items are still excluded from bulk consensus
(for_selection) and the type-to-add dropdown, whose jobs are unchanged.

Frontend: store.dismiss flags in place (canonical tags) rather than dropping;
new store.undismiss reverts. SuggestionItem renders the rejected state and
swaps ✗→↶; ✓ still accepts (which clears the rejection server-side).

Tests: rejected-surfaced-flagged-then-reversible (service) + undismiss
endpoint idempotency (API).

Completes #1134's reversible-rejection half. Heads-as-suggestion-source is
the remaining piece.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

backend/app/api/suggestions.py

@@ -61,6 +61,10 @@ async def get_suggestions(image_id: int):
                         # modal's "Treat as alias"/"Remove alias" affordances.
                         "raw_name": s.raw_name,
                         "via_alias": s.via_alias,
+                        # operator dismissed this tag for this image — surfaced
+                        # (not dropped) so the rail can show it rejected + offer
+                        # one-click un-reject.
+                        "rejected": s.rejected,
                     }
                     for s in items
                 ]
@@ -131,6 +135,21 @@ async def dismiss_suggestion(image_id: int):
     return "", 204
 
 
+@suggestions_bp.route(
+    "/images/<int:image_id>/suggestions/undismiss", methods=["POST"]
+)
+async def undismiss_suggestion(image_id: int):
+    """Reverse a per-image dismissal (reject-recovery). Idempotent — undoing a
+    tag that isn't rejected is a no-op delete."""
+    body = await request.get_json()
+    if not body or "tag_id" not in body:
+        return jsonify({"error": "tag_id required"}), 400
+    async with get_session() as session:
+        await AllowlistService(session).undismiss(image_id, body["tag_id"])
+        await session.commit()
+    return "", 204
+
+
 @suggestions_bp.route("/suggestions/bulk", methods=["POST"])
 async def bulk_suggestions():
     body = await request.get_json()