refactor: finish the jsonutil dedupe; back up config before auto-upgrade

Duplicated helpers
  cmd/rip had its own copies of six helpers that already existed in
  internal/jsonutil: asString (StringFromAny), searchInt (IntFromAny),
  nestedSearchInt and nestedSearchString (NestedAny/NestedString),
  firstNonEmpty (FirstNonEmpty) and firstPositiveInt. firstPositiveInt was
  duplicated a third and fourth time in provider/yandex and provider/beatport,
  so it moves into jsonutil alongside the rest. jsonutil is back to 100%
  coverage.

  searchBool is deliberately left alone, with a comment saying why: it accepts
  only a real bool, and switching it to jsonutil.BoolFromAny would start
  coercing strings and numbers, changing how search results get labelled.

config auto-upgrade is no longer destructive
  UpgradeOutdated re-serializes the whole config from the parsed struct, so any
  key this version does not know about is dropped, and it runs automatically
  from run() without prompting. It now copies the original to
  <path>.bak-<unixtime> (0600, since it holds the same credentials) and
  validates the result before writing. It returns the backup path so the CLI
  can tell the user where it went. The comment states plainly that this is a
  version stamp rather than a migration framework.

cmd/rip tests
  The command bodies are testable now that they are functions returning exit
  codes rather than arms of a switch that called os.Exit. Covers the usage
  errors for every command, the quality floor on `rip id`, config path output,
  the --version flag, and that applyQualityOverride reaches all six sources and
  clamps only the two with a floor. cmd/rip coverage 17% -> 26%.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/config/config.go

@@ -7,6 +7,7 @@ import (
 	"path/filepath"
 	"strings"
 	"sync"
+	"time"
 
 	"github.com/pelletier/go-toml/v2"
 )
@@ -370,25 +371,42 @@ func NormalizeCodec(raw string) (string, error) {
 	}
 }
 
-func UpgradeOutdated(path string) (string, error) {
+// UpgradeOutdated rewrites an outdated config at the current version. It runs
+// automatically, without prompting, so it copies the original aside first: the
+// rewrite re-serializes the whole file from the parsed struct, which drops any
+// key this version does not know about. It returns the config path and the
+// backup path.
+//
+// This is a version stamp, not a migration framework. There are no per-version
+// transforms, so a future key rename still needs explicit handling here.
+func UpgradeOutdated(path string) (resolved string, backup string, err error) {
 	resolvedPath, err := resolvePath(path)
 	if err != nil {
-		return "", err
+		return "", "", err
 	}
 	raw, err := os.ReadFile(resolvedPath)
 	if err != nil {
-		return "", err
+		return "", "", err
 	}
+
+	backupPath := fmt.Sprintf("%s.bak-%d", resolvedPath, time.Now().Unix())
+	if err = os.WriteFile(backupPath, raw, 0o600); err != nil {
+		return "", "", fmt.Errorf("could not back up %s before upgrading: %w", resolvedPath, err)
+	}
+
 	data := DefaultConfigData()
 	if err = toml.Unmarshal(raw, &data); err != nil {
-		return "", err
+		return "", "", err
 	}
 	applyRuntimeDefaults(&data)
 	data.Misc.Version = CurrentConfigVersion
-	if err = saveConfigData(resolvedPath, data); err != nil {
-		return "", err
+	if err = Validate(&data); err != nil {
+		return "", "", err
 	}
-	return resolvedPath, nil
+	if err = saveConfigData(resolvedPath, data); err != nil {
+		return "", "", err
+	}
+	return resolvedPath, backupPath, nil
 }
 
 func (c *Config) SaveFile() error {