Fix completed backfills not showing 100% progress

[petera-block]

Summary

• Backfills with state COMPLETE could show less than 100% progress when the backfilled count didn't exactly match the computed count
• Clamp ProgressBar percentage to 0–100% via .coerceIn(0.0, 100.0)
• Override progress to 100% on both the backfill detail page and the backfills list table when state is COMPLETE

Test plan

• ./gradlew :service:build passes
• Verify on a completed backfill (e.g. #38168) that progress now shows 100%
• Verify in-progress and paused backfills still show accurate partial progress

🤖 Generated with Claude Code

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