chore(tags): remove orphaned cluster tag-gaps route + service method

The cluster tag-gap feature's only UI (Explore's TagGapPanel) was removed in the
3-pane rework, leaving the backend that fed it with no caller. Surgical removal:

- drop the POST /api/images/cluster/tag-gaps route (cluster_tag_gaps)
- drop BulkTagService.tag_gaps (+ the now-unused `import math`)
- drop the tag_gaps tests (test_bulk_tag_service, test_api_bulk_tags)

BulkTagService's common_tags / bulk_add / bulk_remove stay — they still back the
gallery bulk editor. Pure deletion, no behaviour change.

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

backend/app/api/tags.py

@@ -320,27 +320,6 @@ async def common_tags():
     return jsonify({"tags": tags})
 
 
-@tags_bp.route("/images/cluster/tag-gaps", methods=["POST"])
-async def cluster_tag_gaps():
-    """Cluster-consensus tag gaps for a visual neighbour set (#94 Explore):
-    tags on >= threshold of the images but not all. Each gap carries the
-    laggard image ids (minus rejections) for apply-to-cluster."""
-    body = await request.get_json()
-    ids, err = _parse_bulk_ids(body)
-    if err:
-        return err
-    try:
-        threshold = float(body.get("threshold", 0.6))
-    except (TypeError, ValueError):
-        threshold = 0.6
-    threshold = min(1.0, max(0.0, threshold))
-    async with get_session() as session:
-        gaps = await BulkTagService(session).tag_gaps(ids, threshold)
-    return jsonify(
-        {"gaps": gaps, "total": len(set(ids)), "threshold": threshold}
-    )
-
-
 @tags_bp.route("/images/bulk/tags", methods=["POST"])
 async def bulk_add_tag():
     body = await request.get_json()