Add comprehensive tests for OAuth callback flows and update pyproject.toml

- Extended OAuth callback tests to cover various scenarios (e.g., account linking, user creation, inactive users, and token/user info failures).
- Added `app/init_db.py` to the excluded files in `pyproject.toml`.

backend/.coveragerc

@@ -14,6 +14,9 @@ omit =
     app/crud/base_async.py
     app/core/database_async.py
 
+    # CLI scripts - run manually, not tested
+    app/init_db.py
+
     # __init__ files with no logic
     app/__init__.py
     app/api/__init__.py