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/app/models/__init__.py

@@ -0,0 +1,6 @@
+from app.models.user import User
+from app.models.project import Project
+from app.models.edit import Edit
+from app.models.patch import Patch
+
+__all__ = ["User", "Project", "Edit", "Patch"]