Pin DEEPFACE_FACENET512_SHA256 so the model-integrity check is enforced (currently skipped when empty)

Inline code marker.

**Location:** `app/core/config.py:933`

```
TODO: populate with known-good hash once the model file is available. sha256sum ~/.deepface/weights/facenet512_weights.h5 — when empty string, startup logs a WARNING and skips the integrity check (does not raise).
```

The marker stays in the code (annotated `(#this)` in place).

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_Migrated from `app/core/config.py:933`._