fc3k: /api/admin tag endpoints — Tier-B delete + merge + usage-count helper

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/admin.py

@@ -112,3 +112,70 @@ async def images_bulk_delete():
     from ..tasks.admin import bulk_delete_images_task
     async_result = bulk_delete_images_task.delay(image_ids=image_ids)
     return jsonify({"task_id": async_result.id}), 202
+
+
+@admin_bp.route("/tags/<int:tag_id>", methods=["DELETE"])
+async def tag_delete(tag_id: int):
+    """Tier-B sync delete. UI yes/no modal is the only confirmation."""
+    from ..services.cleanup_service import delete_tag
+
+    async with get_session() as session:
+        try:
+            result = await session.run_sync(
+                lambda sync_sess: delete_tag(sync_sess, tag_id=tag_id)
+            )
+        except LookupError:
+            return _bad("not_found", status=404)
+    return jsonify(result)
+
+
+@admin_bp.route("/tags/<int:dest_id>/merge", methods=["POST"])
+async def tag_merge(dest_id: int):
+    """Wraps TagService.merge. Source repoints to dest, dest survives,
+    source row deleted, protective alias auto-created if source was
+    ML-applied or allowlisted."""
+    from ..services.tag_service import TagMergeConflict, TagService, TagValidationError
+
+    body = await request.get_json(silent=True) or {}
+    source_id = body.get("source_id")
+    if not isinstance(source_id, int) or source_id == dest_id:
+        return _bad("invalid_source_id", detail="source_id must be int and differ from dest")
+
+    async with get_session() as session:
+        try:
+            result = await TagService(session).merge(
+                source_id=source_id, target_id=dest_id,
+            )
+        except TagMergeConflict as exc:
+            return _bad("merge_conflict", status=409, detail=str(exc))
+        except TagValidationError as exc:
+            return _bad("tag_kind_mismatch", detail=str(exc))
+        except LookupError:
+            return _bad("not_found", status=404)
+
+    # MergeResult is a frozen dataclass — flatten to dict.
+    return jsonify({
+        "result": {
+            "target_id": result.target_id,
+            "target_name": result.target_name,
+            "target_kind": result.target_kind,
+            "merged_count": result.merged_count,
+            "alias_created": result.alias_created,
+            "source_deleted": result.source_deleted,
+        },
+    })
+
+
+@admin_bp.route("/tags/<int:tag_id>/usage-count", methods=["GET"])
+async def tag_usage_count(tag_id: int):
+    """Helper for the Tier-B yes/no prompt; surfaces "N associations"
+    in the dialog so the operator knows what they're nuking."""
+    from ..services.cleanup_service import count_tag_associations
+
+    async with get_session() as session:
+        count = await session.run_sync(
+            lambda sync_sess: count_tag_associations(
+                sync_sess, tag_id=tag_id,
+            )
+        )
+    return jsonify({"count": count})