Remediate all lint findings under the canonical golangci-lint config

Fix every finding surfaced by the canonical .golangci.yml with
golangci-lint v2.12.2 (refs #61), behavior-preserving throughout:

- err113: dynamic errors replaced with package-level sentinels and %w
  wrapping; direct comparisons converted to errors.Is
- goprintffuncname: printf-style helpers renamed with an f suffix
  (ui.Writer message methods, cli.ReportErrorf, database.Fatalf,
  vaultik stdoutf) and all call sites updated
- revive: stuttering type names renamed (blob.Handler, blob.WithReader,
  blob.ChunkPosition, storage.URL, storage.Info), doc comments added,
  unused parameters blanked, package comments added
- contextcheck/noctx: ctx threaded through blob.Packer
  (AddChunk/Flush/FinalizeBlob/PackChunks) and scanner call sites;
  context-aware exec and sql variants used
- funlen/cyclop/gocognit/nestif/dupl: oversized or duplicated
  functions split into focused helpers across production and test code
- paralleltest/tparallel/thelper/usetesting/testpackage: tests
  parallelized where safe (global log.Initialize kept in the serial
  phase), helpers marked, t.TempDir adopted, external test packages
  where only exported API is used
- gosec: integer conversions clamped or justified, header timeouts
  added, remaining findings suppressed with per-site justifications
- mnd/goconst/lll/wsl_v5/nlreturn/noinlineerr/errcheck and other
  mechanical findings fixed directly

Remove the deprecated log.LogOptions alias (callers migrated to
log.Options). make check is green.

internal/log/log.go

@@ -1,3 +1,5 @@
+// Package log provides the application-wide structured logger: slog
+// with a colorized TTY handler on terminals and JSON output otherwise.
 package log
 
 import (
@@ -12,12 +14,12 @@ import (
 	"golang.org/x/term"
 )
 
-// LogLevel represents the logging level.
-type LogLevel int
+// Level represents the logging level.
+type Level int
 
 const (
 	// LevelFatal represents a fatal error level that will exit the program.
-	LevelFatal LogLevel = iota
+	LevelFatal Level = iota
 	// LevelError represents an error level.
 	LevelError
 	// LevelWarn represents a warning level.
@@ -38,6 +40,7 @@ type Config struct {
 	Quiet   bool
 }
 
+//nolint:gochecknoglobals // package-level logger is the package's purpose
 var logger *slog.Logger
 
 // Initialize sets up the global logger based on the provided configuration.
@@ -45,18 +48,19 @@ func Initialize(cfg Config) {
 	// Determine log level based on configuration
 	var level slog.Level
 
-	if cfg.Cron || cfg.Quiet {
+	switch {
+	case cfg.Cron || cfg.Quiet:
 		// In cron/quiet mode keep warnings and errors visible — the
 		// whole point of --cron is to stay silent only on total
 		// success, so that anything cron emails to root is genuinely
 		// "something went wrong, look at it." A backup with stuck
 		// permission errors or skipped files should NOT be silent.
 		level = slog.LevelWarn
-	} else if cfg.Debug || strings.Contains(os.Getenv("GODEBUG"), "vaultik") {
+	case cfg.Debug || strings.Contains(os.Getenv("GODEBUG"), "vaultik"):
 		level = slog.LevelDebug
-	} else if cfg.Verbose {
+	case cfg.Verbose:
 		level = slog.LevelInfo
-	} else {
+	default:
 		level = slog.LevelWarn
 	}
 
@@ -78,9 +82,13 @@ func Initialize(cfg Config) {
 	slog.SetDefault(logger)
 }
 
+// callerSkipFrames is the number of stack frames between runtime.Caller
+// and the code that invoked the package-level logging function.
+const callerSkipFrames = 2
+
 // getCaller returns the caller information as a string
-func getCaller(skip int) string {
-	_, file, line, ok := runtime.Caller(skip)
+func getCaller() string {
+	_, file, line, ok := runtime.Caller(callerSkipFrames)
 	if !ok {
 		return "unknown"
 	}
@@ -92,7 +100,7 @@ func getCaller(skip int) string {
 func Fatal(msg string, args ...any) {
 	if logger != nil {
 		// Add caller info to args
-		args = append(args, "caller", getCaller(2))
+		args = append(args, "caller", getCaller())
 		logger.Error(msg, args...)
 	}
 
@@ -107,7 +115,7 @@ func Fatalf(format string, args ...any) {
 // Error logs an error message.
 func Error(msg string, args ...any) {
 	if logger != nil {
-		args = append(args, "caller", getCaller(2))
+		args = append(args, "caller", getCaller())
 		logger.Error(msg, args...)
 	}
 }
@@ -120,7 +128,7 @@ func Errorf(format string, args ...any) {
 // Warn logs a warning message.
 func Warn(msg string, args ...any) {
 	if logger != nil {
-		args = append(args, "caller", getCaller(2))
+		args = append(args, "caller", getCaller())
 		logger.Warn(msg, args...)
 	}
 }
@@ -133,7 +141,7 @@ func Warnf(format string, args ...any) {
 // Notice logs a notice message (mapped to Info level).
 func Notice(msg string, args ...any) {
 	if logger != nil {
-		args = append(args, "caller", getCaller(2))
+		args = append(args, "caller", getCaller())
 		logger.Info(msg, args...)
 	}
 }
@@ -146,7 +154,7 @@ func Noticef(format string, args ...any) {
 // Info logs an informational message.
 func Info(msg string, args ...any) {
 	if logger != nil {
-		args = append(args, "caller", getCaller(2))
+		args = append(args, "caller", getCaller())
 		logger.Info(msg, args...)
 	}
 }
@@ -159,7 +167,7 @@ func Infof(format string, args ...any) {
 // Debug logs a debug message.
 func Debug(msg string, args ...any) {
 	if logger != nil {
-		args = append(args, "caller", getCaller(2))
+		args = append(args, "caller", getCaller())
 		logger.Debug(msg, args...)
 	}
 }
@@ -179,7 +187,7 @@ func With(args ...any) *slog.Logger {
 }
 
 // WithContext returns a logger with the provided context.
-func WithContext(ctx context.Context) *slog.Logger {
+func WithContext(_ context.Context) *slog.Logger {
 	return logger
 }