Chore/ts strict mode

[mijinummi]

Pull Request: Enable TypeScript Strict Mode

Overview

This PR implements issue #327 TypeScript Strict Mode.
It enables strict type checking in tsconfig.json and fixes type errors across the backend.

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Changes Introduced

• Enabled strict mode in tsconfig.json.
• Added explicit return types to functions.
• Replaced any with proper types.
• Fixed null/undefined handling.
• Updated interfaces for consistency.

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Acceptance Criteria ✅

• Strict mode enabled
• Type errors fixed
• Functions have return types
• Code compiles cleanly

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Contribution Notes

• Close TypeScript Strict Mode #327