docs(readme): correct outdated deploy docs and docker-compose examples

README had drifted from the codebase: wrong test path, a stale /health
sample response, a docker-compose volume mount pointing at a path the
image never writes to (with no SQLite data volume at all), a missing
CONTRIBUTIONS_RETENTION_DAYS example, and an architecture diagram
predating the SQLite store/refetch command. Also fixes
docker-compose.prod.yml, which was missing the data volume and
CONTRIBUTIONS_RETENTION_DAYS env var present in docker-compose.yml —
without it, a prod deployment on this file loses contribution history
on every container recreate.

docker-compose.prod.yml

@@ -18,9 +18,11 @@ services:
       GITEA_USER: "${GITEA_USER:-}"
       GITEA_TOKEN: "${GITEA_TOKEN:-}"
       GITEA_URL: "${GITEA_URL:-}"
+      CONTRIBUTIONS_RETENTION_DAYS: "${CONTRIBUTIONS_RETENTION_DAYS:-0}"
     volumes:
       - cache:/app/var/cache/prod/pools
       - logs:/app/var/log
+      - data:/app/var/data
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:8080/health"]
       interval: 30s
@@ -30,3 +32,4 @@ services:
 volumes:
   cache:
   logs:
+  data: