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/.env.example

@@ -0,0 +1,21 @@
+# Database
+DATABASE_URL=sqlite:///./data/ai_photo_edit.db
+
+# Security
+SECRET_KEY=your-secret-key-here-change-in-production
+ALGORITHM=HS256
+ACCESS_TOKEN_EXPIRE_MINUTES=30
+
+# AI Provider (configure based on your provider)
+AI_PROVIDER=openai
+OPENAI_API_KEY=your-openai-api-key-here
+# Alternative providers (uncomment as needed)
+# AI_PROVIDER=stability
+# STABILITY_API_KEY=your-stability-api-key-here
+
+# File Storage
+DATA_DIR=/app/data
+MAX_UPLOAD_SIZE_MB=50
+
+# CORS
+CORS_ORIGINS=http://localhost:3000,http://localhost:5173