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>

router/default.conf

@@ -0,0 +1,15 @@
+upstream app {
+    server blue-app:5000;
+}
+
+server {
+    listen 80;
+
+    location / {
+        proxy_pass http://app;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+    }
+}