refactor(phase3-session-b1): extract upscale, control, social, health into sub-routers

Extracted 4 endpoint groups into separate sub-router modules:
- health.py: 1 endpoint (GET /health)
- upscale.py: 1 endpoint (POST /upscale)
- control.py: 2 endpoints (POST /control/process, GET /control/operations)
- social.py: 2 endpoints (POST /social/optimize, GET /social/platforms/{platform}/formats)

__init__.py now composes these sub-routers into the legacy router.
All 33 routes preserved. No functional changes.

backend/routers/image_studio/health.py

@@ -0,0 +1,21 @@
+"""Health check endpoint."""
+
+from fastapi import APIRouter
+
+router = APIRouter(tags=["image-studio"])
+
+
+@router.get("/health", summary="Health Check")
+async def health_check():
+    """Health check endpoint for Image Studio."""
+    return {
+        "status": "healthy",
+        "service": "image_studio",
+        "version": "1.0.0",
+        "modules": {
+            "create_studio": "available",
+            "templates": "available",
+            "providers": "available",
+            "compression": "available",
+        }
+    }