Add health package: REST and gRPC health checks

- Handler(db) returns http.HandlerFunc: 200 ok / 503 unhealthy
- RegisterGRPC registers grpc.health.v1.Health on a gRPC server
- 4 tests: healthy, unhealthy (closed db), content type, gRPC registration

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

health/health_test.go

@@ -0,0 +1,97 @@
+package health
+
+import (
+	"database/sql"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"path/filepath"
+	"testing"
+
+	"google.golang.org/grpc"
+
+	"git.wntrmute.dev/kyle/mcdsl/db"
+)
+
+func TestHandlerHealthy(t *testing.T) {
+	database := openTestDB(t)
+
+	handler := Handler(database)
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/healthz", nil)
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	var body map[string]string
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if body["status"] != "ok" {
+		t.Fatalf("status = %q, want %q", body["status"], "ok")
+	}
+}
+
+func TestHandlerUnhealthy(t *testing.T) {
+	database := openTestDB(t)
+	// Close the database to simulate unhealthy state.
+	_ = database.Close()
+
+	handler := Handler(database)
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/healthz", nil)
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusServiceUnavailable {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusServiceUnavailable)
+	}
+
+	var body map[string]string
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if body["status"] != "unhealthy" {
+		t.Fatalf("status = %q, want %q", body["status"], "unhealthy")
+	}
+	if body["error"] == "" {
+		t.Fatal("expected error message")
+	}
+}
+
+func TestHandlerContentType(t *testing.T) {
+	database := openTestDB(t)
+
+	handler := Handler(database)
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/healthz", nil)
+	handler.ServeHTTP(rec, req)
+
+	if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
+		t.Fatalf("Content-Type = %q, want %q", ct, "application/json")
+	}
+}
+
+func TestRegisterGRPC(t *testing.T) {
+	srv := grpc.NewServer()
+	// Should not panic.
+	RegisterGRPC(srv)
+
+	info := srv.GetServiceInfo()
+	if _, ok := info["grpc.health.v1.Health"]; !ok {
+		t.Fatal("health service not registered")
+	}
+}
+
+func openTestDB(t *testing.T) *sql.DB {
+	t.Helper()
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.db")
+	database, err := db.Open(path)
+	if err != nil {
+		t.Fatalf("db.Open: %v", err)
+	}
+	t.Cleanup(func() { _ = database.Close() })
+	return database
+}