Implement script generation endpoint (Task 18)

- Added ScriptRequest, ScriptResponse, and ScriptSegment Pydantic models
- Created /api/tts/script POST endpoint in main.py
- Integrated with TTSService to generate script segments using AI
- Added generate_script_segments method to TTSService
- Returns segments with unique IDs for frontend tracking
- Handles errors gracefully when AI segmentation fails
- Falls back to pattern-based segmentation when Ollama unavailable
- Added comprehensive tests for the endpoint
- Validates: Requirements 15.1, 15.2

backend/main.py

@@ -34,6 +34,21 @@ class RolesResponse(BaseModel):
     """Response model for role detection endpoint."""
     roles: List[RoleInfo] = Field(..., description="List of detected roles")
 
+class ScriptSegment(BaseModel):
+    """Model representing a script segment with speaker assignment."""
+    id: str = Field(..., description="Unique segment identifier")
+    speaker: str = Field(..., description="Assigned speaker/role")
+    text: str = Field(..., description="Text content for this segment")
+
+class ScriptRequest(BaseModel):
+    """Request model for script generation endpoint."""
+    text: str = Field(..., description="Text to segment and analyze")
+    roles: List[str] = Field(..., description="List of detected role IDs")
+
+class ScriptResponse(BaseModel):
+    """Response model for script generation endpoint."""
+    segments: List[ScriptSegment] = Field(..., description="List of script segments with speaker assignments")
+
 class RoleVoiceMapping(BaseModel):
     """Model for role-to-voice mapping."""
     id: str = Field(..., description="Role identifier")
@@ -129,6 +144,95 @@ async def detect_roles(request: RoleDetectionRequest):
         logger.error(f"Error during role detection: {str(e)}", exc_info=True)
         raise
 
+@app.post("/api/tts/script", response_model=ScriptResponse)
+async def generate_script(request: ScriptRequest):
+    """
+    Generates script preview with AI-segmented text and speaker assignments.
+    
+    Uses AI (Ollama) to intelligently segment the text and assign speakers to each segment.
+    Falls back to pattern-based segmentation if AI is unavailable.
+    
+    Args:
+        request: ScriptRequest containing text and list of detected roles
+        
+    Returns:
+        ScriptResponse with list of segments, each containing id, speaker, and text
+        
+    Raises:
+        400: If validation fails (empty text or roles)
+        500: If script generation fails
+    """
+    logger.info(
+        f"Script generation requested for text of length {len(request.text)} "
+        f"with {len(request.roles)} roles"
+    )
+    
+    # Validate text is not empty
+    if not request.text or not request.text.strip():
+        logger.error("Empty text provided for script generation")
+        return JSONResponse(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            content={
+                "error": "ValidationError",
+                "message": "Text field cannot be empty",
+                "details": {"field": "text"}
+            }
+        )
+    
+    # Validate roles list is not empty
+    if not request.roles:
+        logger.error("No roles provided for script generation")
+        return JSONResponse(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            content={
+                "error": "ValidationError",
+                "message": "At least one role is required",
+                "details": {"field": "roles"}
+            }
+        )
+    
+    try:
+        # Generate script segments using TTS service
+        segments = await tts_service.generate_script_segments(
+            text=request.text,
+            roles=request.roles
+        )
+        
+        if not segments:
+            logger.warning("Script generation produced no segments")
+            return JSONResponse(
+                status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+                content={
+                    "error": "ScriptGenerationError",
+                    "message": "Failed to generate script segments",
+                    "details": {"reason": "No segments produced"}
+                }
+            )
+        
+        # Convert to response format with unique IDs
+        script_segments = [
+            ScriptSegment(
+                id=f"seg_{i}",
+                speaker=speaker,
+                text=text
+            )
+            for i, (speaker, text) in enumerate(segments)
+        ]
+        
+        logger.info(f"Generated script with {len(script_segments)} segments")
+        return ScriptResponse(segments=script_segments)
+        
+    except Exception as e:
+        logger.error(f"Error during script generation: {str(e)}", exc_info=True)
+        return JSONResponse(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            content={
+                "error": "ScriptGenerationError",
+                "message": "Failed to generate script",
+                "details": {"error": str(e)}
+            }
+        )
+
 @app.post("/api/tts")
 async def generate_audiobook(request: TTSRequest):
     """