fix(deploy): move router config write to after health check passes

Router had no profile so it was always included in `up -d --wait`.
Writing the new target's config BEFORE the wait caused the router to become
unhealthy if the new slot failed — leaving it in a broken state for the next
deploy attempt.

Now: router keeps its old config (pointing to the still-running old slot)
during the health check wait, so it stays healthy throughout. Config is only
written and nginx -s reload triggered after the new slot passes its health
check. This is the correct blue-green pattern.

Also add `retries: 3` and `start_period: 10s` to the router health check
for resilience against transient startup failures.

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

docker-compose.prod.yml

@@ -14,6 +14,8 @@ services:
       test: ["CMD", "nginx", "-t"]
       interval: 30s
       timeout: 5s
+      retries: 3
+      start_period: 10s
 
   litestream:
     image: litestream/litestream:latest