Unified Queue: add DB migration for existing pending activities

[mna]

#25682

Checklist for submitter

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
  See Changes files for more information.
• If database migrations are included, checked table schema to confirm autoupdate
• For database migrations:
  • Checked schema for all modified table for columns that will auto-update timestamps during migration.
  • Confirmed that updating the timestamps is acceptable, and will not cause unwanted side effects.
• Added/updated automated tests
• Manual QA for all new/changed functionality

[codecov]

Codecov Report

Attention: Patch coverage is 87.85942% with 38 lines in your changes missing coverage. Please review.

Project coverage is 63.85%. Comparing base (bf41766) to head (98bedae).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...20250217093329_MigratePendingUpcomingActivities.go	87.85%	26 Missing and 12 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26413      +/-   ##
==========================================
+ Coverage   63.79%   63.85%   +0.05%     
==========================================
  Files        1656     1657       +1     
  Lines      158749   159062     +313     
  Branches     4057     4093      +36     
==========================================
+ Hits       101277   101568     +291     
- Misses      49544    49557      +13     
- Partials     7928     7937       +9     

Flag	Coverage Δ
backend	64.68% <87.85%> (+0.06%)	⬆️

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[getvictor]

LGTM

[getvictor]

👍