Show placeholder user reassignment deadline and disable undo action after expiration

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Background

As part of the data retention policy implementation #527002, placeholder reassignment will only be allowed for 1 year. Group owners need visibility of this deadline and should be prevented from undoing keep-as-placeholder actions after the expiration period.

Requirements

UI Updates

• Add deadline information to the Placeholder User table showing when the reassignment period expires
• Disable the "Undo keep placeholder" button when the reassignment period has expired

Backend Changes

• Update GraphQL API to include deadline/expiration information
• Modify GraphQL mutation to prevent undoing keep-as-placeholder action after the expiration period

Acceptance Criteria

• Deadline is visible in the Placeholder User table
• "Undo keep placeholder" button is disabled for expired placeholder users
• GraphQL API returns deadline information for placeholder users
• GraphQL mutation blocks undo actions for expired placeholder users
• Group owners can undo keep-as-placeholder actions only within the one-year period