Turn the remerger into a service

[rdettai-sk]

Description

Nothing fancy, just run everynight at 4am.

How was this PR tested?

Describe how you tested this PR.

[Copilot]

Pull request overview

This PR adjusts mature-merge (“remerger”) configuration so that, when no explicit day-span is provided, the system attempts mature merges successively across multiple timestamp day spans (0, 1, then 2), and updates CLI parsing/help + unit tests accordingly.

Changes:

• Change split_timestamp_days_range from u8 to Option<u8> and default it to None to enable multi-pass day-span attempts.
• Add a day-span candidate sequence (0/1/2) when unset and run mature merge passes per candidate.
• Update merge planning to require an explicit day-span (tests updated to set Some(0) / Some(1) explicitly) and update CLI arg behavior/documentation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
quickwit/quickwit-indexing/src/mature_merge.rs	Makes day-span optional, adds multi-pass execution over candidate day spans, and updates the per-index merge entrypoint signature/tests.
quickwit/quickwit-indexing/src/mature_merge_plan.rs	Requires an explicit day-span during planning and updates tests to pass Some(days) explicitly.
quickwit/quickwit-cli/src/tool.rs	Updates CLI help text and parsing to support optional day-span config (unset => multi-pass behavior).

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