9.5 upgrade & add underflow protection for referred volume (backport #3261)

[mergify]

Changelist

• Add 9.5 upgrade and underflow protection for ExpireOldStats

Test Plan

• Unit tests

Author/Reviewer Checklist

• If this PR has changes that result in a different app state given the same prior state and transaction list, manually add the state-breaking label.
• If the PR has breaking postgres changes to the indexer add the indexer-postgres-breaking label.
• If this PR isn't state-breaking but has changes that modify behavior in PrepareProposal or ProcessProposal, manually add the label proposal-breaking.
• If this PR is one of many that implement a specific feature, manually label them all feature:[feature-name].
• If you wish to for mergify-bot to automatically create a PR to backport your change to a release branch, manually add the label backport/[branch-name].
• Manually add any of the following labels: refactor, chore, bug.

Summary by CodeRabbit

• New Features

  • v9.5 upgrade: migrates 30-day referred volumes into epoch-level statistics and registers a v9.5 upgrade handler.

• Bug Fixes

  • Expiration logic now guards against numeric underflow when trimming old stats.

• Tests

  • Added unit and container-style tests for the v9.5 migration (empty and multi-user scenarios); removed legacy v9.4 container test.

• Chores

  • Bumped test/version files to v9.5.

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This is an automatic backport of pull request #3261 done by [Mergify](https://mergify.com).

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