feat(api): add /health endpoint with Docker healthchecks

Allow Docker to distinguish a running container from one actually
serving traffic by adding a /health endpoint that pings the database.
Returns 200 when healthy, 503 when the DB connection is lost.

Add HEALTHCHECK to Dockerfile and healthcheck config to the server
service in docker-compose.yml. Also add start_period to the existing
db healthcheck for consistency.

docker/docker-compose.yml

@@ -20,6 +20,7 @@ services:
       test: ["CMD-SHELL", "pg_isready -U postgres"]
       interval: 5s
       timeout: 5s
+      start_period: 5s
       retries: 5
   pgadmin:
     image: dpage/pgadmin4
@@ -59,3 +60,9 @@ services:
       - "54006:54006"
       - "54007:54007"
       - "54008:54008"
+    healthcheck:
+      test: ["CMD", "wget", "-qO-", "http://localhost:8080/health"]
+      interval: 10s
+      timeout: 3s
+      start_period: 15s
+      retries: 3