🧪 testing improvement: add unit tests for upload-excel api and fix failing tests

[deitaur]

🎯 What:

• The endpoint POST /api/upload-excel lacked any unit testing, leaving crucial logic regarding importing user inventories from Excel vulnerable to regressions.
• Previous tests in api.test.js were completely broken due to the ensureAuthenticated checking req.user from Passport.js, which was undefined since we didn't mock Passport appropriately before importing app.

📊 Coverage:

• Added robust mocks for pg connection pools/clients to intercept BEGIN, COMMIT, ROLLBACK and query execution.
• Added tests for POST /api/upload-excel that dynamically create a real Excel document in buffer to test the exceljs loading logic properly.
• Covered the missing file condition (returning 400).
• Covered the database exception condition to test correct transaction rollback implementation (returning 500).
• Globally mocked Express app.request.isAuthenticated and app.request.user to easily mock authenticated sessions for supertest.

✨ Result:

• Total tests in the suite successfully passes (5 out of 5 tests). Test coverage greatly improved for critical upload paths and no more tests failing out of the box due to passport and missing .env mock parameters.

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PR created automatically by Jules for task 14958525485771934442 started by @deitaur

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