feat: initial SaaS scaffold

- domain: User, Workspace, Project, AssetLibrary, Asset, IngestJob
- ports: repository interfaces
- application: use cases
- adapters: in-memory
- API: FastAPI 5 routes
- worker: Celery ingest_asset
- tests: 4 passing

.gitignore

@@ -0,0 +1,36 @@
+# Node / frontend
+node_modules/
+.next/
+out/
+dist/
+coverage/
+
+# Python / backend
+.venv/
+venv/
+__pycache__/
+.pytest_cache/
+.mypy_cache/
+.pytype/
+ruff_cache/
+*.pyc
+
+# Env / secrets
+.env
+.env.*
+!.env.example
+
+# OS / editor
+.DS_Store
+Thumbs.db
+.vscode/
+.idea/
+
+# Logs / temp
+*.log
+tmp/
+temp/
+
+# Build / runtime artifacts
+build/
+.runtime/