Make Zoekt storage buffer factor dynamic

What does this MR do and why?

Replaces the hardcoded buffer_factor: 3 in PlanningService with an instance-wide dynamic ratio computed from aggregate observed Zoekt disk usage vs source repository size across all ready indices.

On GitLab.com the observed ratio is ~0.5x the source repository size, meaning nodes are significantly over-reserved with the current 3x hardcoded value. This change makes the planner converge toward actual usage patterns over time.

Implementation

• Search::Zoekt::Index.global_buffer_factor aggregates SUM(used_storage_bytes) / SUM(repository_size) across all ready indices joined to root storage statistics, applies a 1.2x safety margin, and falls back to 3.0x when no data is available.
• Result is cached via Rails.cache for 1 hour to avoid expensive repeated aggregation.
• Gated behind zoekt_dynamic_buffer_factor feature flag (gitlab_com_derisk, default off).
• Factor is now visible in gitlab:zoekt:info rake task output.

Database query

https://console.postgres.aiundefined/shared/714b111e-62b0-44e9-87e0-5e56b1c23cec

SELECT
    SUM(per_namespace_stats.total_used)::float / NULLIF (SUM(per_namespace_stats.repository_size), 0) AS ratio
FROM (
    SELECT
        zoekt_enabled_namespaces.root_namespace_id,
        SUM(zoekt_indices.used_storage_bytes) AS total_used,
        MAX(namespace_root_storage_statistics.repository_size) AS repository_size
    FROM
        zoekt_indices
        INNER JOIN zoekt_enabled_namespaces ON zoekt_enabled_namespaces.id = zoekt_indices.zoekt_enabled_namespace_id
        INNER JOIN namespace_root_storage_statistics ON namespace_root_storage_statistics.namespace_id = zoekt_enabled_namespaces.root_namespace_id
    WHERE
        zoekt_indices.state = 10
        AND zoekt_indices.used_storage_bytes > 1024
        AND namespace_root_storage_statistics.repository_size > 0
    GROUP BY
        zoekt_enabled_namespaces.root_namespace_id) per_namespace_stats

MR acceptance checklist

• Tests added
• Feature flag added
• Documentation updated

Closes #592180 (closed)