Add backend unit test coverage

[devin-ai-integration]

Summary

The backend had no collectable automated tests, leaving every production module at 0% measured coverage. Add 56 isolated unit tests around app setup, configuration validation, OAuth/session behavior, playlist routes, Gemini response handling, and Spotify API requests; configure pytest-cov with a 90% floor (current total: 95%).

Also import requests in the playlist routes, which the new search and playlist-fetch tests exposed as a missing runtime dependency.

Link to Devin session: https://app.devin.ai/sessions/58659a7dd5dc457194fd556eb3547d22
Requested by: @praneels2005

[devin-ai-integration]

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