Schedule knowledge graph indexing

What does this MR do and why?

Schedules knowledge graph indexing

• when a default branch is updated, a graph indexing task is created
• it adds two services:
  • IndexingTaskService - responsible for creating a new indexing task on one of replicas
  • ProvisionService - responsible for creating required number of replicas on most suitable nodes. "replica" == represents a copy of graph for a namespace on a zoekt node
• if there is no knowledge graph replica yet, a new one is created. Only one replica is created by default (the plan is to copy over update graph between replicas which will be done when graph db is created)
• a zoekt node with biggest available space is used for the new replica
• if there is already an indexing tasks (in pending status) for the namespace, we skip adding another one

References

Related to #540850 (closed)

How to set up and validate locally

1. In rails console enable the KG indexing

   Feature.enable(:knowledge_graph_indexing)

2. Test creation of indexing tasks for a project:

   ns = Project.find(7).project_namespace
   ::Ai::KnowledgeGraph::IndexingTaskService.new(ns.id, :index_graph_repo).execute

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