feat: implement DOB-based age calculation

Overview

This MR replaces the usage of age with Date of Birth (DOB) and introduces dynamic age calculation. Additionally, it includes a migration strategy to handle existing users who previously had only age stored.

What does this MR do and why?

Previously, patient age was stored directly, which could become inaccurate over time.

This MR:

• Introduces DOB as the primary field
• Computes age dynamically from DOB
• Migrates existing users from age → DOB

This ensures:

• Accurate and consistent data
• Backward compatibility for existing users
• Smooth transition without data loss

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Changes Made

• Removed direct dependency on age

• Added date_of_birth field

• Implemented age calculation utility from DOB

• Added DOB validation (valid date, no future dates, 0–150 range)

• Migration logic for existing users:

  • Converted stored age → approximate DOB
  • Ensured no null/invalid states

• Updated API responses to return computed patient_age

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Technical Details

• Age is no longer stored; it is derived from DOB

• For existing users:

  • DOB is generated using:

    DOB = current_date - age (years)

  • This is an approximation (exact birthdate unknown)

• Migration ensures:

  • No breaking changes for old data
  • All users now have DOB

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Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• ♻️ Refactor

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How to Validate Locally

1. Check old users:

   • Ensure DOB is populated after migration

2. Create new user with DOB:

   • Verify validation works

3. Fetch user:

   • Confirm age is correctly calculated

4. Compare:

   • Old users → migrated DOB
   • New users → actual DOB

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Testing Done

• Unit tests added/updated
• API endpoint tests passing

Test Cases Covered:

Scenario	Expected Result	Status
Old user migration	DOB generated correctly	✅
Valid DOB input	Accepted	✅
Future DOB	Rejected	✅
Age calculation	Accurate	✅

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Known Limitations / Technical Debt

• Migrated DOB is approximate, not exact
• Future improvement: allow users to update DOB manually

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Additional Notes

• Ensures backward compatibility
• Smooth migration without breaking existing records
• Aligns backend fully with DOB-based system

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