Add SAM (Segment Anything) via Replicate API and update Docker

- Implement SAM object selection via Replicate API
  - Click on any object to select it with AI precision
  - Falls back to flood-fill if Replicate API unavailable
- Update Dockerfile for rembg dependencies
  - Add required system libraries (libsm6, libxext6, etc)
  - Pre-download rembg model during build
  - Create data directories for models and patches

backend/Dockerfile

@@ -2,10 +2,15 @@ FROM python:3.11-slim
 
 WORKDIR /app
 
-# Install system dependencies
+# Install system dependencies for OpenCV, rembg, and image processing
 RUN apt-get update && apt-get install -y \
     libgl1 \
     libglib2.0-0 \
+    libsm6 \
+    libxext6 \
+    libxrender-dev \
+    libgomp1 \
+    wget \
     && rm -rf /var/lib/apt/lists/*
 
 # Copy requirements
@@ -14,11 +19,14 @@ COPY requirements.txt .
 # Install Python dependencies
 RUN pip install --no-cache-dir -r requirements.txt
 
+# Pre-download rembg model (u2net) to avoid first-run delay
+RUN python -c "from rembg import remove; print('rembg model downloaded')" || true
+
 # Copy application
 COPY . .
 
-# Create data directory
-RUN mkdir -p /app/data/projects
+# Create data directories
+RUN mkdir -p /app/data/projects /app/data/patches /app/data/models
 
 # Expose port
 EXPOSE 8000