Add unit tests for src/backend/server/server.go

[hanneshapke]

src/backend/server/server.go (~28KB) handles HTTP request orchestration but has no server_test.go next to it. The rest of the backend (pii/, proxy/, providers/, config/) has companion test files, so the server is an outlier.

What to add

Create src/backend/server/server_test.go covering at least:

• Happy-path request handling for one provider (OpenAI is fine)
• Malformed JSON body — should return 400
• Empty body — should return 400
• Missing/invalid Authorization header pass-through
• Concurrent request handling (table-driven with t.Parallel())

Hints

• Use httptest.NewRecorder() / httptest.NewServer() — see src/backend/proxy/handler_test.go for the pattern already in use here.
• Stub or fake the PII detector + provider — you don't need a real ONNX model.
• Run with make test-all or go test ./src/backend/server/....

Difficulty: good first issue, ~1–2 hours.