fix(services): Improve status code consistency

[vprashrex]

Issue

Closes #834

Summary

• Before: Error handling was generic and lacked specific exceptions.
• Now: Implemented domain-specific exceptions for better granularity and traceability in error handling.
• Updated error logging to include exception details.
• Modified HTTP status codes in tests and service methods to conform to new conventions.

Checklist

Before submitting a pull request, please ensure that you mark these task.

• Ran fastapi run --reload app/main.py or docker compose up in the repository root and test.
• If you've fixed a bug or added code that is tested and has test cases.

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[github-actions]

OpenAPI changes   ⚪ No API surface changes

Note

This PR does not modify the API contract.

_{main ↔ 1a3e74a9 · generated by oasdiff}

[codecov]

Codecov Report

❌ Patch coverage is 86.16601% with 35 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/app/crud/config/version.py	11.11%	8 Missing ⚠️
backend/app/crud/config/config.py	12.50%	7 Missing ⚠️
backend/app/crud/credentials.py	58.33%	5 Missing ⚠️
backend/app/crud/evaluations/embeddings.py	50.00%	5 Missing ⚠️
backend/app/services/evaluations/fast.py	44.44%	5 Missing ⚠️
backend/app/core/exception_handlers.py	81.81%	2 Missing ⚠️
...ckend/app/api/routes/stt_evaluations/evaluation.py	0.00%	1 Missing ⚠️
backend/app/crud/evaluations/batch.py	93.75%	1 Missing ⚠️
backend/app/crud/evaluations/core.py	50.00%	1 Missing ⚠️

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