[bug] Strategy cache hash should not depend solely on the file itself

Changing the strategy in request obviously doesn't change anything when the cache is used (even if OCR was performed using a different strategy).

- [ ] Analyze which fields could critically interfere with the results (I guess it's `file_hash + strategy_name + language`).
- [ ] Combine them (it's not necessary to hash them after proper validation; it could be `language:strategy_name:file_hash`).