chore: initial project setup with backend, frontend, and infrastructure

Add complete NexaPantry application structure including:
- Docker Compose configuration with PostgreSQL, Redis, FastAPI backend, worker, frontend and Caddy
- Environment configuration template with database, auth, and service settings
- GitHub Actions CI workflow for backend/frontend linting, testing, auditing and Docker builds
- AGPL-3.0 license and comprehensive README with setup, development, and security documentation
- Backend

docs/architecture.md

@@ -0,0 +1,15 @@
+# Architecture
+
+NexaPantry uses a small but explicit service architecture:
+
+- React/Vite/TypeScript frontend with TailwindCSS and PWA service worker
+- FastAPI backend with SQLAlchemy ORM
+- PostgreSQL as the system of record
+- Redis for future distributed rate limiting and queued job coordination
+- Caddy as reverse proxy and security header layer
+- Worker process for expiry notifications and mail delivery preparation
+
+Core authorization is home-scoped. Every product, shopping item and recipe request first checks the caller's home membership. Instance admins can manage system settings and users through dedicated admin endpoints.
+
+External barcode lookup is hidden behind `ProductLookup`, currently implemented by `OpenFoodFactsLookup`. A future provider can be added without changing the frontend contract.
+