feat(explore): cluster-consensus tag-gaps service + route (#94a)

Cluster C, milestone #94. BulkTagService.tag_gaps(image_ids, threshold) finds
tags applied to >= threshold fraction of a visual neighbour set but not all of
it (the '7 of 10 share Miku; these 3 don't' signal). Each gap carries the
laggard image ids minus any TagSuggestionRejection rows, so apply-to-cluster
never re-proposes a tag a neighbour dismissed. 100%-common tags and <2-image
sets are excluded. New POST /api/images/cluster/tag-gaps.

Tests: consensus found / common excluded / missing ids; rejected laggard
excluded from missing; tag dropped when all laggards rejected; <2 images empty;
route shape + bad input.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XCUHUGQLrBrkgyk1t49kpX

backend/app/api/tags.py

@@ -320,6 +320,27 @@ 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()