feat: initialize cart app with blueprints, templates, and CI

Extract cart, order, and orders blueprints with their service layer,
templates, Dockerfile (APP_MODULE=app:app, IMAGE=cart), entrypoint,
and Gitea CI workflow from the coop monolith.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

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+# Cart App
+
+Shopping cart, checkout, and order management service for the Rose Ash cooperative marketplace.
+
+## Overview
+
+This is the **cart** microservice, split from the Rose Ash monolith. It handles:
+
+- **Shopping cart** - Add/remove products, view cart, cart summary API
+- **Checkout** - SumUp payment integration with hosted checkout
+- **Orders** - Order listing, detail view, payment status tracking
+- **Calendar bookings** - Calendar entry cart items and checkout integration
+
+## Architecture
+
+- **Framework:** Quart (async Flask)
+- **Database:** PostgreSQL 16 via SQLAlchemy 2.0 (async)
+- **Payments:** SumUp Hosted Checkout
+- **Frontend:** HTMX + Jinja2 templates + Tailwind CSS
+
+## Directory Structure
+
+```
+app.py                  # Quart application factory
+bp/
+  cart/                 # Cart blueprint (add, view, checkout, webhooks)
+    routes.py
+    api.py              # Internal API (server-to-server, CSRF-exempt)
+    login_helper.py     # Cart merge on login
+    services/           # Business logic layer
+  order/                # Single order detail blueprint
+    routes.py
+    filters/qs.py       # Query string helpers
+  orders/               # Order listing blueprint
+    routes.py
+    filters/qs.py
+templates/
+  _types/cart/          # Cart templates
+  _types/order/         # Single order templates
+  _types/orders/        # Order listing templates
+entrypoint.sh           # Docker entrypoint (migrations + server start)
+Dockerfile              # Container build
+.gitea/workflows/ci.yml # CI/CD pipeline
+```
+
+## Running
+
+```bash
+# Set environment variables
+export APP_MODULE=app:app
+export DATABASE_URL_ASYNC=postgresql+asyncpg://user:pass@localhost/coop
+export REDIS_URL=redis://localhost:6379/0
+export SECRET_KEY=your-secret-key
+
+# Run the server
+hypercorn app:app --reload --bind 0.0.0.0:8002
+```
+
+## Cross-App Communication
+
+The cart app exposes internal API endpoints at `/internal/cart/` for other services:
+
+- `GET /internal/cart/summary` - Cart count and total for the current session/user
+- `POST /internal/cart/adopt` - Adopt anonymous cart items after user login
+
+## Docker
+
+```bash
+docker build -t cart:latest .
+docker run -p 8002:8000 --env-file .env cart:latest
+```