Implement backend API foundation

- Created FastAPI application with CORS middleware
- Defined Pydantic models (RoleDetectionRequest, RolesResponse, TTSRequest, ErrorResponse)
- Implemented /api/health endpoint
- Set up error handling middleware for validation and general exceptions
- Configured logging with INFO level
- Added test script to verify API functionality
- Validates Requirements: 9.1, 9.2, 9.4

backend/main.py

@@ -1,10 +1,59 @@
 """
 FastAPI backend for Text to Audiobook application.
 """
-from fastapi import FastAPI
+import logging
+from typing import List, Optional
+from fastapi import FastAPI, Request, status
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import JSONResponse
+from fastapi.exceptions import RequestValidationError
+from pydantic import BaseModel, Field
 
-app = FastAPI(title="Text to Audiobook API")
+# Configure logging
+logging.basicConfig(
+    level=logging.INFO,
+    format='%(asctime)s - %(name)s - %(levelname)s - %(message)s'
+)
+logger = logging.getLogger(__name__)
+
+# Pydantic Models
+class RoleDetectionRequest(BaseModel):
+    """Request model for role detection endpoint."""
+    text: str = Field(..., description="Text to analyze for role detection")
+
+class RoleInfo(BaseModel):
+    """Model representing a detected role."""
+    id: str = Field(..., description="Unique identifier for the role")
+    label: str = Field(..., description="Display name for the role")
+
+class RolesResponse(BaseModel):
+    """Response model for role detection endpoint."""
+    roles: List[RoleInfo] = Field(..., description="List of detected roles")
+
+class RoleVoiceMapping(BaseModel):
+    """Model for role-to-voice mapping."""
+    id: str = Field(..., description="Role identifier")
+    voice: str = Field(..., description="Kokoro voice ID")
+
+class TTSRequest(BaseModel):
+    """Request model for TTS generation endpoint."""
+    text: str = Field(..., description="Text to convert to speech")
+    lang: str = Field(..., description="Language code (a, b, f, j)")
+    roles: List[RoleVoiceMapping] = Field(..., description="Role to voice mappings")
+    speed: Optional[float] = Field(1.0, description="Playback speed (0.5-1.5)")
+
+class ErrorResponse(BaseModel):
+    """Standard error response model."""
+    error: str = Field(..., description="Error type")
+    message: str = Field(..., description="Human-readable error message")
+    details: Optional[dict] = Field(None, description="Additional error details")
+
+# Initialize FastAPI app
+app = FastAPI(
+    title="Text to Audiobook API",
+    description="API for converting text to multi-voice audiobooks using Kokoro TTS",
+    version="1.0.0"
+)
 
 # Configure CORS
 app.add_middleware(
@@ -15,8 +64,36 @@ app.add_middleware(
     allow_headers=["*"],
 )
 
+# Error handling middleware
+@app.exception_handler(RequestValidationError)
+async def validation_exception_handler(request: Request, exc: RequestValidationError):
+    """Handle validation errors with custom error response."""
+    logger.error(f"Validation error: {exc.errors()}")
+    return JSONResponse(
+        status_code=status.HTTP_400_BAD_REQUEST,
+        content={
+            "error": "ValidationError",
+            "message": "Invalid request data",
+            "details": {"errors": exc.errors()}
+        }
+    )
 
+@app.exception_handler(Exception)
+async def general_exception_handler(request: Request, exc: Exception):
+    """Handle general exceptions with custom error response."""
+    logger.error(f"Unhandled exception: {str(exc)}", exc_info=True)
+    return JSONResponse(
+        status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+        content={
+            "error": "InternalServerError",
+            "message": "An unexpected error occurred",
+            "details": {"error": str(exc)}
+        }
+    )
+
+# API Endpoints
 @app.get("/api/health")
 async def health_check():
     """Health check endpoint."""
+    logger.info("Health check requested")
     return {"status": "ok"}