[#793] feat: update feasible value quantile from 90th to 80th

[wayangalihpratama]

Summary

Updated the "feasible value" calculation logic across the stack to use the 80th quantile instead of the 90th. This change ensures that the targets displayed in Step 2 are more realistically achievable for farmer segments based on the uploaded data.

Key Changes

• Backend: Updated case_import_process_confirmed_segmentation.py to calculate feasible_value using values.quantile(0.8).
• Frontend: Updated the tooltip in EnterIncomeData.js to inform users that feasibility is now calculated as the 80th quantile.
• Documentation: Synchronized CASE_MANAGEMENT_UX.md and updated GEMINI.md.

Verification

• Unit Tests: Backend unit tests and a targeted mathematical test for quantile(0.8) passed.
• Linting: Frontend linting (yarn lint) passed.
• Full Check: Full backend check (./check.sh) passed.

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