Zoekt project search should select correct zoekt node for replica search

What does this MR do and why?

With the introduction of replicas the project should have has_many relations with the zoekt_repositories. Override the zoekt_nodes method to select the node that has a ready repository for the project search.

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SELECT
    "zoekt_nodes".*
FROM
    "zoekt_nodes"
    INNER JOIN "zoekt_indices" ON "zoekt_indices"."zoekt_node_id" = "zoekt_nodes"."id"
    INNER JOIN "zoekt_repositories" ON "zoekt_repositories"."zoekt_index_id" = "zoekt_indices"."id"
WHERE
    "zoekt_nodes"."last_seen_at" >= '2024-10-04 18:47:37.413989'
    AND "zoekt_repositories"."state" = 10
    AND "zoekt_repositories"."project_id" = 278964

https://console.postgres.ai/shared/a26df723-8f75-49dd-9b02-8efa3e8264c7

Related to #494418 (closed)