Add repo_to_index check for zoekt scheduling service

What does this MR do and why?

Change the behavior of InitialIndexingEventWorker.

Before: Calls the IndexingTaskService on each project of a namespace which will create the indexing tasks for all the projects except empty projects. And move the index to initializing.

After: Create the pending Search::Zoekt::Repository records for all projects of a namespace. And move the index to initializing.

Create a new scheduling task repo_to_index_check that will perform the indexing for all pending or initializing zoekt_repositories. And move the zoekt_repository to initializing if the project is non empty repo and move to ready if the project is empty.

MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Query Plan

https://console.postgres.ai/shared/53a78e47-3645-4483-b611-26936af4e3ed

SELECT
    "zoekt_repositories"."id"
FROM
    "zoekt_repositories"
WHERE
    "zoekt_repositories"."state" IN (0, 1)
    AND "zoekt_repositories"."id" >= 79
ORDER BY
    "zoekt_repositories"."id" ASC
LIMIT 1 OFFSET 1000

Related to #500350 (closed)