Move ai-stack and paintplus vendored source under vendor/

Matches the existing vendor/easy-asterisk convention (used by
services/asterisk.sh) instead of two one-off top-level directories that
cluttered the repo root and didn't look like anything else next to
setup.sh, lib/, services/, extras/. Only the two services' own SRC_DIR
path resolution and header comments needed updating — nothing else in
the repo referenced the old ./ai-stack / ./paintplus paths.

Also documents vendor/ in README.md's Layout section.

vendor/paintplus/backend/Dockerfile

@@ -0,0 +1,37 @@
+FROM python:3.11-slim
+
+WORKDIR /app
+
+# Install system dependencies for OpenCV, SAM, and image processing
+RUN apt-get update && apt-get install -y \
+    libgl1 \
+    libglib2.0-0 \
+    libsm6 \
+    libxext6 \
+    libxrender-dev \
+    libgomp1 \
+    wget \
+    git \
+    && rm -rf /var/lib/apt/lists/*
+
+# Copy requirements
+COPY requirements.txt .
+
+# Install Python dependencies
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy application
+COPY . .
+
+# Copy entrypoint script and make it executable
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+
+# Create data directories
+RUN mkdir -p /app/data/projects /app/data/patches /app/data/models
+
+# Expose port
+EXPOSE 8000
+
+# Use entrypoint script (auto-populates eyes on first run, then starts server)
+ENTRYPOINT ["/entrypoint.sh"]