Add Dockerfile, Compose stacks, and /healthz endpoint

Dockerfile is multi-stage:
- build: golang:1.25-bookworm, CGO_ENABLED=0 (modernc.org/sqlite
  is pure-Go) + -trimpath + -ldflags "-s -w" so the resulting
  binary is small and reproducible-ish.
- runtime: gcr.io/distroless/static:nonroot, ~2 MB. Runs as uid
  65532. /data and /var/lib/librenotes are declared volumes so
  per-tenant notes and the SQLite database survive container
  restarts.

healthcheck subcommand: distroless static has no shell or
wget/curl, so /healthz is reachable but no client to call it. A
new "librenotes healthcheck" subcommand uses net/http to GET
$LIBRENOTES_HEALTHCHECK_URL (default 127.0.0.1:8080/healthz) and
exits non-zero on failure. Both compose files invoke it from the
HEALTHCHECK directive.

httpapi adds a tiny GET /healthz that returns {"status":"ok"}
(no DB ping yet — added when readiness probes need it).

docker-compose.yml: dev stack on :8080 with named volumes and a
LogMailer; everything via env vars, JWT secret defaulted to a
dev value.
docker-compose.prod.yml: layered overrides — pulls a registry
image, expects LIBRENOTES_* env, sets memory limits and JSON-
file log rotation.

Closes #25.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/httpapi/router.go

@@ -22,6 +22,10 @@ type Server struct {
 func (s *Server) Routes() http.Handler {
 	mux := http.NewServeMux()
 
+	mux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"status":"ok"}`))
+	})
 	mux.HandleFunc("/auth/login", s.Auth.HandleLogin)
 	mux.HandleFunc("/auth/verify", s.Auth.HandleVerify)