Add a health check for the daemon and RPC endpoint (#34)

* Add a health check for the daemon and RPC endpoint

SeafileClient.is_healthy() checks seaf-cli status and makes a real RPC
call (get_repo_list) against the daemon's Unix socket - neither needs
the Seafile server, so it works without a token or password.

healthcheck.py exposes it as a plain script for Docker's HEALTHCHECK.
Each check is a fresh process spawned by the Docker daemon as root,
unlike the long-lived start.py process, so it drops to the seafile
user with runuser before touching the RPC socket - the one place that
mechanism still belongs after the earlier in-process privilege drop.

Verified against the real image: healthcheck.py exits 0 once the real
daemon is up (through the actual main() entrypoint, not a test
fixture) and nonzero when no daemon has been started.

* Assert the return value in the daemon-down health check test

Copilot review: the test checked the RPC call was skipped but not
what is_healthy() actually returned in that case.

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>

tests/smoke.sh

@@ -119,6 +119,42 @@ else
 fi
 docker rm -f dsc-smoke >/dev/null
 
+echo "== the HEALTHCHECK command through the real entrypoint"
+# Runs through main(), unlike the fixture above: HOME/UID/GID come from the
+# same drop_privileges() path a real deployment uses, so .ccnet ends up
+# under /dsc like it does in production, not under root's home directory.
+docker rm -f dsc-smoke >/dev/null 2>&1 || true
+docker run -d $PLATFORM_ARG --name dsc-smoke \
+    -e SERVER_HOST=example.invalid -e USERNAME=u -e PASSWORD=p -e LIBRARY_ID=x \
+    "$IMAGE" >/dev/null
+
+ready=0
+for _ in $(seq 1 30); do
+    if docker logs dsc-smoke 2>&1 | grep -q "Seafile daemon is ready"; then ready=1; break; fi
+    sleep 1
+done
+
+if [ "$ready" -eq 0 ]; then
+    fail "the daemon never became ready, HEALTHCHECK not measured"
+else
+    if docker exec dsc-smoke runuser -u seafile -- python3 /dsc/healthcheck.py; then
+        pass "HEALTHCHECK command exits 0 while the daemon is up"
+    else
+        fail "HEALTHCHECK command did not exit 0 while the daemon was up"
+    fi
+fi
+docker rm -f dsc-smoke >/dev/null
+
+echo "== the HEALTHCHECK command reports unhealthy when the daemon is down"
+docker rm -f dsc-smoke >/dev/null 2>&1 || true
+docker run -d $PLATFORM_ARG --name dsc-smoke "$IMAGE" sleep 60 >/dev/null
+if docker exec dsc-smoke runuser -u seafile -- python3 /dsc/healthcheck.py; then
+    fail "HEALTHCHECK command exited 0 with no daemon running"
+else
+    pass "HEALTHCHECK command exits nonzero with no daemon running"
+fi
+docker rm -f dsc-smoke >/dev/null
+
 echo "== configuration errors stop the container with a useful message"
 out=$(docker run --rm $PLATFORM_ARG -e SERVER_HOST=h -e USERNAME=u -e LIBRARY_ID=x "$IMAGE" 2>&1; echo "rc=$?")
 check "no credentials exits 2" "rc=2" "$(echo "$out" | tail -1)"