Cap aggregate forecast previews at question close time

[SylvainChevalier]

Summary

Fix the default aggregate forecast ("latest" CP) preview for questions that were resolved retroactively (resolved as of a past date but kept accumulating forecasts until the resolution was entered). The preview now correctly reflects the forecast value at the question's effective close time rather than the most recent forecast.

Key Changes

• questions/services/forecasts.py: Updated get_last_aggregated_forecasts_for_questions() to filter aggregate forecasts by actual_close_time. For open questions (where actual_close_time is null), the most recent forecast is used. For closed/resolved questions, only forecasts that started at or before actual_close_time are considered.

• questions/serializers/aggregate_forecasts.py: Added _get_latest_aggregate_forecast() helper function that selects the appropriate forecast for the "latest" CP preview. It respects the actual_close_time cutoff for closed questions while maintaining full history in serialization.

• tests/unit/test_questions/test_services/test_forecasts.py: Added comprehensive test suite (TestCPAtResolutionTime) covering:

  • Retroactively resolved questions with forecasts after resolution time
  • Verification that the latest preview uses the forecast at resolution time, not the most recent one
  • Confirmation that open questions still use the most recent forecast

Implementation Details

• actual_close_time is defined as min(actual_resolve_time, scheduled_close_time), matching the horizon scoring logic
• The solution maintains backward compatibility: open questions continue to use the most recent forecast
• Full forecast history is preserved in serialization; only the "latest" preview is capped at close time
• Includes fallback behavior: if all forecasts start after the cutoff (edge case), the earliest available forecast is used to keep a CP visible

https://claude.ai/code/session_01XgUzpLZNHiVXK3t7UGoSsU

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[SylvainChevalier]

Superseded by #5055. This PR's branch name (claude/github-issue-5053-sb5kii) contains "github", which Fly.io's abuse filter blocks for the preview-deploy app name. The identical change has been recreated on claude/issue-5053-sb5kii in #5055. Closing this one.

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