Backport of Fix partition missing error in project_daily_statistics backfill (18.5)

What does this MR do and why?

Related: PostgreSQL error when upgrading from 18.5.3 -> ... (#582526) BackfillPartitionedProjectDailyStatistics error... (#582512 - closed)

This MR fixes partition errors when Self-managed/Dedicated customers upgrade to GitLab 18.5

Problem:

• Migration has hardcoded START_DATE = Date.new(2025, 8, 1)
• Model has retain_for: 3.months
• For GitLab.com (upgraded Aug): August within 3 months, works
• For self-managed (upgrading Dec): August 4 months old, partition deleted by partition manager
• Backfill tries to copy August data -> ERROR: no partition found

Root cause: Hardcoded START_DATE assumes controlled rollout timing, breaks for self-managed deployments upgrading months later.

Solution (three-part fix):

1. Dynamic START_DATE: Use 3.months.ago.beginning_of_month instead of hardcoded date to respect retention policy at migration time
2. Filter in background job: Skip records outside retention window during backfill to handle already-enqueued jobs on affected instances
3. Removed retain_for: Prevent partition manager from deleting old partitions while backfilling data

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
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  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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