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/storage/url.go

@@ -7,8 +7,26 @@ import (
 	"strings"
 )
 
-// StorageURL represents a parsed storage URL.
-type StorageURL struct {
+// Storage URL scheme names.
+const (
+	schemeFile   = "file"
+	schemeS3     = "s3"
+	schemeRclone = "rclone"
+)
+
+// Sentinel errors for storage URL parsing.
+var (
+	ErrEmptyStorageURL   = errors.New("storage URL is empty")
+	ErrEmptyFilePath     = errors.New("file URL path is empty")
+	ErrMissingBucket     = errors.New("s3 URL missing bucket name")
+	ErrMissingRemote     = errors.New("rclone URL missing remote name")
+	ErrUnsupportedScheme = errors.New(
+		"unsupported URL scheme: must start with s3://, file://, or rclone://")
+	ErrUnsupportedStorage = errors.New("unsupported storage scheme")
+)
+
+// URL represents a parsed storage URL.
+type URL struct {
 	Scheme       string // "s3", "file", or "rclone"
 	Bucket       string // S3 bucket name (empty for file/rclone)
 	Prefix       string // Path within bucket or filesystem base path
@@ -23,20 +41,20 @@ type StorageURL struct {
 //   - s3://bucket/prefix?endpoint=host&region=us-east-1&ssl=true
 //   - file:///absolute/path/to/backup
 //   - rclone://remote/path/to/backups
-func ParseStorageURL(rawURL string) (*StorageURL, error) {
+func ParseStorageURL(rawURL string) (*URL, error) {
 	if rawURL == "" {
-		return nil, errors.New("storage URL is empty")
+		return nil, ErrEmptyStorageURL
 	}
 
 	// Handle file:// URLs
 	if after, ok := strings.CutPrefix(rawURL, "file://"); ok {
 		path := after
 		if path == "" {
-			return nil, errors.New("file URL path is empty")
+			return nil, ErrEmptyFilePath
 		}
 
-		return &StorageURL{
-			Scheme: "file",
+		return &URL{
+			Scheme: schemeFile,
 			Prefix: path,
 		}, nil
 	}
@@ -50,7 +68,7 @@ func ParseStorageURL(rawURL string) (*StorageURL, error) {
 
 		bucket := u.Host
 		if bucket == "" {
-			return nil, errors.New("s3 URL missing bucket name")
+			return nil, ErrMissingBucket
 		}
 
 		prefix := strings.TrimPrefix(u.Path, "/")
@@ -62,8 +80,8 @@ func ParseStorageURL(rawURL string) (*StorageURL, error) {
 			useSSL = false
 		}
 
-		return &StorageURL{
-			Scheme:   "s3",
+		return &URL{
+			Scheme:   schemeS3,
 			Bucket:   bucket,
 			Prefix:   prefix,
 			Endpoint: query.Get("endpoint"),
@@ -81,27 +99,27 @@ func ParseStorageURL(rawURL string) (*StorageURL, error) {
 
 		remote := u.Host
 		if remote == "" {
-			return nil, errors.New("rclone URL missing remote name")
+			return nil, ErrMissingRemote
 		}
 
 		path := strings.TrimPrefix(u.Path, "/")
 
-		return &StorageURL{
-			Scheme:       "rclone",
+		return &URL{
+			Scheme:       schemeRclone,
 			Prefix:       path,
 			RcloneRemote: remote,
 		}, nil
 	}
 
-	return nil, errors.New("unsupported URL scheme: must start with s3://, file://, or rclone://")
+	return nil, ErrUnsupportedScheme
 }
 
 // String returns a human-readable representation of the storage URL.
-func (u *StorageURL) String() string {
+func (u *URL) String() string {
 	switch u.Scheme {
-	case "file":
+	case schemeFile:
 		return "file://" + u.Prefix
-	case "s3":
+	case schemeS3:
 		endpoint := u.Endpoint
 		if endpoint == "" {
 			endpoint = "s3.amazonaws.com"
@@ -112,7 +130,7 @@ func (u *StorageURL) String() string {
 		}
 
 		return fmt.Sprintf("s3://%s (endpoint: %s)", u.Bucket, endpoint)
-	case "rclone":
+	case schemeRclone:
 		if u.Prefix != "" {
 			return fmt.Sprintf("rclone://%s/%s", u.RcloneRemote, u.Prefix)
 		}