fix: Rename TranscriptionCallback to STTEngineCallback to avoid conflict

ISTTEngine.hpp and ISTTService.hpp both defined TranscriptionCallback
with different signatures:
- ISTTEngine: void(const std::string& text)
- ISTTService: void(const std::string& text, STTMode mode)

This caused compilation errors due to conflicting declarations.

Solution: Rename the low-level engine callback to STTEngineCallback
to distinguish it from the high-level service callback.

Fixes: Compilation errors in Phase 7.1 STT Service Layer
- All 120 unit tests passing (282 assertions)
- Build successful

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/shared/audio/ISTTEngine.hpp

@@ -8,9 +8,9 @@
 namespace aissia {
 
 /**
- * @brief Callback for transcription results
+ * @brief Callback for transcription results (low-level engine)
  */
-using TranscriptionCallback = std::function<void(const std::string& text)>;
+using STTEngineCallback = std::function<void(const std::string& text)>;
 
 /**
  * @brief Interface for Speech-to-Text engines