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/run.go

@@ -9,6 +9,15 @@ import (
 	"time"
 )
 
+// cleanup removes a temporary file, logging a warning if removal fails so
+// that leaked scratch files are surfaced rather than silently ignored. A
+// missing file is not an error.
+func cleanup(path string) {
+	if err := os.Remove(path); err != nil && !os.IsNotExist(err) {
+		slog.Warn("failed to remove temporary file", "path", path, "error", err)
+	}
+}
+
 func Run(targetDates []time.Time) error {
 	snapshotDir, snapshotDate, err := FindLatestDailySnapshot()
 	if err != nil {
@@ -100,7 +109,7 @@ func Run(targetDates []time.Time) error {
 		if err := ExtractDay(dstDB, extractedDB, targetDay); err != nil {
 			if errors.Is(err, ErrNoPosts) {
 				slog.Warn("no posts found, skipping day", "date", dayStr)
-				os.Remove(extractedDB)
+				cleanup(extractedDB)
 				skipped++
 				continue
 			}
@@ -109,7 +118,7 @@ func Run(targetDates []time.Time) error {
 
 		// Dump to SQL and compress
 		if err := os.MkdirAll(outputDir, 0755); err != nil {
-			os.Remove(extractedDB)
+			cleanup(extractedDB)
 			return fmt.Errorf("creating output directory %s: %w", outputDir, err)
 		}
 
@@ -117,35 +126,35 @@ func Run(targetDates []time.Time) error {
 
 		slog.Info("dumping and compressing", "tmp_output", outputTmp)
 		if err := DumpAndCompress(extractedDB, outputTmp); err != nil {
-			os.Remove(outputTmp)
-			os.Remove(extractedDB)
+			cleanup(outputTmp)
+			cleanup(extractedDB)
 			return fmt.Errorf("dump and compress for %s: %w", dayStr, err)
 		}
 
 		slog.Info("verifying compressed output")
 		if err := VerifyOutput(outputTmp); err != nil {
-			os.Remove(outputTmp)
-			os.Remove(extractedDB)
+			cleanup(outputTmp)
+			cleanup(extractedDB)
 			return fmt.Errorf("verification failed for %s: %w", dayStr, err)
 		}
 
 		// Atomic rename to final path
 		slog.Info("renaming to final output", "from", outputTmp, "to", outputFinal)
 		if err := os.Rename(outputTmp, outputFinal); err != nil {
-			os.Remove(outputTmp)
-			os.Remove(extractedDB)
+			cleanup(outputTmp)
+			cleanup(extractedDB)
 			return fmt.Errorf("atomic rename for %s: %w", dayStr, err)
 		}
 
 		info, err := os.Stat(outputFinal)
 		if err != nil {
-			os.Remove(extractedDB)
+			cleanup(extractedDB)
 			return fmt.Errorf("stat final output: %w", err)
 		}
 		slog.Info("day completed", "date", dayStr, "path", outputFinal, "size_bytes", info.Size())
 
 		// Remove extracted DB to reclaim space immediately
-		os.Remove(extractedDB)
+		cleanup(extractedDB)
 		processed++
 	}