refactor: update test fixtures and remove unused environment variables

[grillazz]

No description provided.

[Copilot]

Pull request overview

This PR refactors the test fixture infrastructure to improve test isolation and database transaction handling. It introduces a new db_session fixture with explicit transaction management and updates test files to use direct database manipulation where appropriate.

Changes:

• Introduced per-test db_session fixture with transaction rollback for improved test isolation
• Updated test fixtures to use explicit dependency overrides with the new session management
• Removed unused POSTGRES_TEST_USER environment variable from configuration
• Disabled SQL echo logging for the test engine to reduce test output verbosity

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/conftest.py	Refactored test fixtures: added db_session fixture with transaction management, made start_db autouse, updated client fixture to use dependency override with db_session
tests/api/test_stuff.py	Updated tests to accept db_session parameter and use direct database manipulation instead of HTTP calls for test data setup
app/database.py	Disabled SQL echo logging (echo=False) for test engine to reduce verbosity during test execution
app/config.py	Removed unused POSTGRES_TEST_USER field from Settings class
.env	Removed unused POSTGRES_TEST_USER environment variable

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