refactor(phase3-session-b2): extract edit and face_swap into sub-routers

Extracted 2 endpoint groups into separate sub-router modules:
- edit.py: 4 endpoints (POST /edit/process, GET /edit/operations, GET /edit/models, POST /edit/recommend)
- face_swap.py: 3 endpoints (POST /face-swap/process, GET /face-swap/models, POST /face-swap/recommend)

All 33 routes preserved (10 extracted in B1, 7 extracted in B2, 16 remaining in legacy).

backend/routers/image_studio/face_swap.py

@@ -0,0 +1,89 @@
+"""Face Swap Studio endpoints."""
+
+from typing import Dict, Any, Optional
+from fastapi import APIRouter, Depends, HTTPException
+
+from .models import (
+    FaceSwapRequest, FaceSwapResponse, FaceSwapModelsResponse,
+    FaceSwapModelRecommendationRequest, FaceSwapModelRecommendationResponse,
+)
+from .deps import get_studio_manager, _require_user_id
+from services.image_studio import ImageStudioManager
+from services.image_studio.face_swap_service import FaceSwapStudioRequest
+from middleware.auth_middleware import get_current_user
+from utils.logger_utils import get_service_logger
+
+logger = get_service_logger("api.image_studio")
+router = APIRouter(tags=["image-studio"])
+
+
+@router.post("/face-swap/process", response_model=FaceSwapResponse, summary="Process Face Swap")
+async def process_face_swap(
+    request: FaceSwapRequest,
+    current_user: Dict[str, Any] = Depends(get_current_user),
+    studio_manager: ImageStudioManager = Depends(get_studio_manager),
+):
+    """Process face swap request with auto-detection and model selection."""
+    try:
+        user_id = _require_user_id(current_user, "face swap")
+        face_swap_request = FaceSwapStudioRequest(
+            base_image_base64=request.base_image_base64,
+            face_image_base64=request.face_image_base64,
+            model=request.model,
+            target_face_index=request.target_face_index,
+            target_gender=request.target_gender,
+            options=request.options,
+        )
+        result = await studio_manager.face_swap(face_swap_request, user_id=user_id)
+        return FaceSwapResponse(**result)
+    except HTTPException:
+        raise
+    except Exception as e:
+        logger.error(f"[Face Swap] ❌ Error: {e}", exc_info=True)
+        raise HTTPException(status_code=500, detail=f"Face swap failed: {e}")
+
+
+@router.get("/face-swap/models", response_model=FaceSwapModelsResponse, summary="List available face swap models")
+async def get_face_swap_models(
+    tier: Optional[str] = None,
+    current_user: Dict[str, Any] = Depends(get_current_user),
+    studio_manager: ImageStudioManager = Depends(get_studio_manager),
+):
+    """Get available WaveSpeed face swap models with metadata.
+
+    Query Parameters:
+    - tier: Filter by tier ("budget", "mid", "premium")
+    """
+    try:
+        result = studio_manager.get_face_swap_models(tier=tier)
+        return FaceSwapModelsResponse(**result)
+    except Exception as e:
+        logger.error(f"[Face Swap Models] ❌ Error: {str(e)}", exc_info=True)
+        raise HTTPException(status_code=500, detail="Failed to load face swap models")
+
+
+@router.post("/face-swap/recommend", response_model=FaceSwapModelRecommendationResponse, summary="Get face swap model recommendation")
+async def recommend_face_swap_model(
+    request: FaceSwapModelRecommendationRequest,
+    current_user: Dict[str, Any] = Depends(get_current_user),
+    studio_manager: ImageStudioManager = Depends(get_studio_manager),
+):
+    """Get recommended face swap model based on image resolutions and user preferences.
+
+    Auto-detects best model when user doesn't specify one.
+    """
+    try:
+        user_tier = request.user_tier
+        if not user_tier and current_user:
+            user_tier = current_user.get("tier") or current_user.get("subscription_tier")
+
+        result = studio_manager.recommend_face_swap_model(
+            base_image_resolution=request.base_image_resolution,
+            face_image_resolution=request.face_image_resolution,
+            user_tier=user_tier,
+            preferences=request.preferences,
+        )
+        return FaceSwapModelRecommendationResponse(**result)
+    except Exception as e:
+        logger.error(f"[Face Swap Recommend] ❌ Error: {str(e)}", exc_info=True)
+        raise HTTPException(status_code=500, detail=f"Failed to get recommendation: {e}")