Fix errcheck lint failures with proper error handling

Handle every error return flagged by golangci-lint's errcheck rather
than discarding it:

- run.go: add a cleanup() helper that logs a warning (and ignores
  ErrNotExist) when removing a temp file fails, so leaked scratch files
  are surfaced; use it for all best-effort removals.
- copy.go / extract.go: log a warning on deferred Close() failures for
  the source file, destination DB, and result-set rows.
- extract.go: on the rollback path, ignore the benign sql.ErrTxDone
  (already committed) and log any other rollback failure.
- verify.go: add killCat() which ignores os.ErrProcessDone (zstdcat
  already exited via SIGPIPE) and logs any unexpected kill failure.

make check is clean (0 lint issues, tests pass).

internal/bsdaily/verify.go

@@ -1,12 +1,26 @@
 package bsdaily
 
 import (
+	"errors"
 	"fmt"
 	"log/slog"
+	"os"
 	"os/exec"
 	"strings"
 )
 
+// killCat terminates the zstdcat process, ignoring the benign case where it
+// has already exited (e.g. after receiving SIGPIPE when head closed the pipe)
+// and logging any other failure.
+func killCat(cmd *exec.Cmd) {
+	if cmd.Process == nil {
+		return
+	}
+	if err := cmd.Process.Kill(); err != nil && !errors.Is(err, os.ErrProcessDone) {
+		slog.Warn("failed to kill zstdcat process", "error", err)
+	}
+}
+
 func VerifyOutput(path string) error {
 	slog.Info("running zstdmt integrity check")
 	testCmd := exec.Command("zstdmt", "--test", path)
@@ -34,18 +48,18 @@ func VerifyOutput(path string) error {
 		return fmt.Errorf("starting zstdcat: %w", err)
 	}
 	if err := headCmd.Start(); err != nil {
-		catCmd.Process.Kill() // Clean up if head fails to start
+		killCat(catCmd) // Clean up if head fails to start
 		return fmt.Errorf("starting head: %w", err)
 	}
 
 	// Wait for head first (it will exit when it has enough lines)
 	if err := headCmd.Wait(); err != nil {
-		catCmd.Process.Kill()
+		killCat(catCmd)
 		return fmt.Errorf("head command failed: %w", err)
 	}
 
 	// Kill zstdcat since head closed the pipe (expected SIGPIPE)
-	catCmd.Process.Kill()
+	killCat(catCmd)
 	_ = catCmd.Wait() // Reap the process
 
 	content := headOut.String()