fix(test): gracefully skip DB tests when database is unreachable

pgxpool.New is lazy and doesn't connect immediately. Add pool.Ping()
after creation so CI environments without PostgreSQL skip cleanly
instead of failing with os.Exit(1).

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

server/internal/handler/handler_test.go

@@ -41,6 +41,11 @@ func TestMain(m *testing.M) {
 		fmt.Printf("Skipping tests: could not connect to database: %v\n", err)
 		os.Exit(0)
 	}
+	if err := pool.Ping(ctx); err != nil {
+		fmt.Printf("Skipping tests: database not reachable: %v\n", err)
+		pool.Close()
+		os.Exit(0)
+	}
 
 	queries := db.New(pool)
 	hub := realtime.NewHub()