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

caddy/Caddyfile

@@ -0,0 +1,19 @@
+:80 {
+  encode zstd gzip
+
+  header {
+    X-Content-Type-Options nosniff
+    X-Frame-Options DENY
+    Referrer-Policy strict-origin-when-cross-origin
+    Permissions-Policy "camera=(self), geolocation=(), microphone=()"
+    Content-Security-Policy "default-src 'self'; img-src 'self' data: https://images.openfoodfacts.org; connect-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; object-src 'none'; base-uri 'self'; frame-ancestors 'none'; form-action 'self'; upgrade-insecure-requests"
+  }
+
+  handle_path /api/* {
+    reverse_proxy backend:8000
+  }
+
+  handle {
+    reverse_proxy frontend:8080
+  }
+}