Build and test with a supported Go release (closes #107)

The test and build stages move from golang:1.24-bookworm, a release
line that no longer gets security fixes, to golang:1.27.2-bookworm,
pinned by digest. The go directive in go.mod moves from 1.24.4 to
1.27.1, the Go in the pinned lint image, so no stage downloads another
toolchain. One test starts its goroutine with sync.WaitGroup.Go, which
the linter asks for once the module targets Go 1.25 or later.

Model: opus-5-5

internal/database/database_internal_test.go

@@ -564,11 +564,8 @@ func TestBatchWriteDuringCheckpoint(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 
 	var wg sync.WaitGroup
-	wg.Add(1)
-
-	go func() {
-		defer wg.Done()
 
+	wg.Go(func() {
 		for {
 			select {
 			case <-ctx.Done():
@@ -577,7 +574,7 @@ func TestBatchWriteDuringCheckpoint(t *testing.T) {
 				_ = db.Checkpoint(ctx) // errors are the checkpoint's own to absorb
 			}
 		}
-	}()
+	})
 
 	ts := time.Now().UTC()
 	for i := range contentionIterations {