fix: add per-user goals limit guard

[Sammmyyyyyyy]

Summary

Adds a per-user goals limit guard to prevent unbounded database row creation and reduce performance issues caused by excessively large goal payloads.

Closes #698

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

Changes Made

• Added MAX_GOALS_PER_USER limit
• Added a count check before inserting new goals
• Added error handling for goal limit verification
• Added .limit(MAX_GOALS_PER_USER) safeguard to the GET query

How to Test

Steps for the reviewer to verify this works:

1. Start the development server using npm run dev
2. Create goals normally and verify creation works below the limit
3. Attempt to create more than 20 goals for the same user
4. Verify the API returns an error once the limit is exceeded
5. Verify the GET route only fetches up to the configured limit

Screenshots (if UI change)

N/A

Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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[github-actions]

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