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/docker-compose.dev.yml

@@ -0,0 +1,46 @@
+version: '3.8'
+
+services:
+  backend:
+    build:
+      context: ./backend
+      dockerfile: Dockerfile
+    container_name: paintplus-backend-dev
+    ports:
+      - "8000:8000"
+    volumes:
+      - ./data:/app/data
+      - ./backend:/app
+    environment:
+      - DATABASE_URL=sqlite:///./data/ai_photo_edit.db
+      - SECRET_KEY=dev-secret-key-change-in-production
+      - AI_PROVIDER=${AI_PROVIDER:-mock}
+      - OPENAI_API_KEY=${OPENAI_API_KEY:-}
+      - STABILITY_API_KEY=${STABILITY_API_KEY:-}
+      - CORS_ORIGINS=http://localhost:5173,http://localhost:3000,http://localhost
+    command: uvicorn app.main:app --host 0.0.0.0 --port 8000 --reload
+    restart: unless-stopped
+    networks:
+      - paintplus-network
+
+  frontend:
+    build:
+      context: ./frontend
+      dockerfile: Dockerfile.dev
+    container_name: paintplus-frontend-dev
+    ports:
+      - "5173:5173"
+    volumes:
+      - ./frontend:/app
+      - /app/node_modules
+    environment:
+      - VITE_API_BASE_URL=http://localhost:8000
+    depends_on:
+      - backend
+    restart: unless-stopped
+    networks:
+      - paintplus-network
+
+networks:
+  paintplus-network:
+    driver: bridge