Add index on runs.finished_at

[stuartc]

Description

This PR adds an index on runs.finished_at to fix database timeouts in the UsageTracking ReportWorker.

We were seeing daily Sentry errors where the worker timed out after 70+ seconds:

Postgrex.Protocol disconnected: client timed out because it queued and checked out the connection for longer than 70000ms

The WorkflowMetricsService queries runs by finished_at date range, but there was no index on that column - causing full table scans.

Validation steps

1. Index is already deployed and working in production
2. Stats show 161k index scans over 12 days with 25M tuples read
3. Timeout errors reduced from daily to 1 in the last two weeks

Additional notes for the reviewer

The migration uses create_if_not_exists and concurrently: true - it's already been applied manually to production.

AI Usage

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• Learning or fact checking
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• Optimisation / refactoring
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Pre-submission checklist

• I have performed a self-review of my code.
• I have implemented and tested all related authorization policies.
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.30%. Comparing base (9e63d12) to head (b173e8e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4308      +/-   ##
==========================================
- Coverage   89.33%   89.30%   -0.03%     
==========================================
  Files         425      425              
  Lines       19913    19913              
==========================================
- Hits        17789    17784       -5     
- Misses       2124     2129       +5     

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