add CI/CD pipeline with blue-green deployment

GitLab CI runs pytest + ruff on master/MRs, then auto-deploys via SSH.
Blue-green strategy using Docker Compose profiles with an nginx router
on port 5000 for zero-downtime switching between slots.

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

CHANGELOG.md

@@ -20,6 +20,11 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
   - `tests/test_billing_routes.py` — route tests (pricing, checkout, manage, cancel, resume, subscription_required decorator)
   - Added `hypothesis>=6.100.0` and `respx>=0.22.0` to dev dependencies for property-based testing and httpx mocking
   - **Factored into Copier template** — all billing tests now generate as `.jinja` templates with provider-specific conditionals for Stripe, Paddle, and LemonSqueezy
+- GitLab CI/CD pipeline (`.gitlab-ci.yml`) — runs pytest + ruff on master/MRs, auto-deploys on master
+- Blue-green deployment with Docker Compose profiles (`docker-compose.prod.yml`, `deploy.sh`)
+  - nginx router on port 5000 proxies to active blue/green slot
+  - Zero-downtime: new slot health-checked before traffic switch
+  - Automatic rollback on failed health check
 
 ### Changed
 - `planner.js` no longer contains `calc()`, `pmt()`, or `calcIRR()` functions — computation moved server-side