Add unit tests for src/backend/pii/masking_service.go

[hanneshapke]

src/backend/pii/masking_service.go orchestrates detection + generator lookup + mapping persistence, but lacks a dedicated masking_service_test.go. Sibling files (database.go, generators, detectors) all have tests.

What to add

Create src/backend/pii/masking_service_test.go covering:

• A string containing 2–3 PII types — each is masked, original/masked pairs are stored in the mapper
• Repeated identical PII — should map to the same dummy value within a session
• No PII in input — output equals input, no mappings stored
• Mixed-language / unicode input — does not panic and round-trips correctly

Hints

• Stub the detector to return a fixed []Entity rather than running ONNX in tests.
• Reuse database_test.go patterns.
• Run go test ./src/backend/pii/....

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