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/requirements.txt

@@ -0,0 +1,25 @@
+fastapi==0.109.0
+uvicorn[standard]==0.27.0
+python-multipart==0.0.6
+Pillow>=10.0.0,<11.0.0
+numpy<2.0.0
+sqlalchemy==2.0.25
+python-jose[cryptography]==3.3.0
+passlib[bcrypt]==1.7.4
+python-dotenv==1.0.0
+aiofiles==23.2.1
+httpx==0.26.0
+pydantic==2.5.3
+pydantic-settings==2.1.0
+email-validator==2.1.0
+opencv-python-headless>=4.10.0
+# SAM (Segment Anything) for smart object selection - runs locally, no API needed
+torch==2.1.2
+torchvision==0.16.2
+segment-anything @ git+https://github.com/facebookresearch/segment-anything.git
+
+# Local AI inpainting — LaMa model (auto GPU/CPU, no API key needed)
+simple-lama-inpainting
+
+# Background removal — rembg enabled now that opencv 4.10+ supports numpy 2.x
+rembg[gpu]