chore: enforce UPPER_SNAKE_CASE constant naming convention

[mijinummi]

Pull Request: Constant Naming Convention

Overview

This PR implements issue #329 Constant Naming Convention.
It enforces consistent UPPER_SNAKE_CASE naming for all constants across the backend codebase.

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Changes Introduced

• Audited all constants in src/**/*.ts.
• Renamed constants to UPPER_SNAKE_CASE.
• Updated references throughout the codebase.
• Added ESLint rule to enforce naming convention.

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Acceptance Criteria ✅

• Constants properly named
• Code compiles and tests pass
• Linting enforces convention

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Contribution Notes

• Close Constant Naming Convention #329
• Assignment required before PR submission
• Timeframe: 48–72 hours
• PR description must include: Close #issue no
• ⭐ Star the repo
• Refer to backend README 🚀

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Checklist Before Merge

• Code reviewed
• Tests passed locally
• Linting verified

Closes #329