Add production-ready HTTP server package with routing, health checks, and middleware

Introduces server/ sub-package as the server-side companion to the existing Client.
Includes Router (over http.ServeMux with groups and mounting), graceful shutdown with
signal handling, health endpoints (/healthz, /readyz), and built-in middlewares
(RequestID, Recovery, Logging). Zero external dependencies.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/health_test.go

@@ -0,0 +1,90 @@
+package server_test
+
+import (
+	"encoding/json"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"git.codelab.vc/pkg/httpx/server"
+)
+
+func TestHealthHandler(t *testing.T) {
+	t.Run("liveness always returns 200", func(t *testing.T) {
+		h := server.HealthHandler()
+
+		w := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, "/healthz", nil)
+		h.ServeHTTP(w, req)
+
+		if w.Code != http.StatusOK {
+			t.Fatalf("got status %d, want %d", w.Code, http.StatusOK)
+		}
+
+		var resp map[string]any
+		if err := json.NewDecoder(w.Body).Decode(&resp); err != nil {
+			t.Fatalf("decode failed: %v", err)
+		}
+		if resp["status"] != "ok" {
+			t.Fatalf("got status %q, want %q", resp["status"], "ok")
+		}
+	})
+
+	t.Run("readiness returns 200 when all checks pass", func(t *testing.T) {
+		h := server.HealthHandler(
+			func() error { return nil },
+			func() error { return nil },
+		)
+
+		w := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, "/readyz", nil)
+		h.ServeHTTP(w, req)
+
+		if w.Code != http.StatusOK {
+			t.Fatalf("got status %d, want %d", w.Code, http.StatusOK)
+		}
+	})
+
+	t.Run("readiness returns 503 when a check fails", func(t *testing.T) {
+		h := server.HealthHandler(
+			func() error { return nil },
+			func() error { return errors.New("db down") },
+		)
+
+		w := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, "/readyz", nil)
+		h.ServeHTTP(w, req)
+
+		if w.Code != http.StatusServiceUnavailable {
+			t.Fatalf("got status %d, want %d", w.Code, http.StatusServiceUnavailable)
+		}
+
+		var resp map[string]any
+		if err := json.NewDecoder(w.Body).Decode(&resp); err != nil {
+			t.Fatalf("decode failed: %v", err)
+		}
+		if resp["status"] != "unavailable" {
+			t.Fatalf("got status %q, want %q", resp["status"], "unavailable")
+		}
+		errs, ok := resp["errors"].([]any)
+		if !ok || len(errs) != 1 {
+			t.Fatalf("expected 1 error, got %v", resp["errors"])
+		}
+		if errs[0] != "db down" {
+			t.Fatalf("got error %q, want %q", errs[0], "db down")
+		}
+	})
+
+	t.Run("readiness returns 200 with no checkers", func(t *testing.T) {
+		h := server.HealthHandler()
+
+		w := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, "/readyz", nil)
+		h.ServeHTTP(w, req)
+
+		if w.Code != http.StatusOK {
+			t.Fatalf("got status %d, want %d", w.Code, http.StatusOK)
+		}
+	})
+}