feat(watcher): log auth field changes on reload

Cache parsed auth contents and compute redacted diffs for prefix, proxy_url,
and disabled when auth files are added or updated.

internal/watcher/diff/auth_diff.go

@@ -0,0 +1,44 @@
+// auth_diff.go computes human-readable diffs for auth file field changes.
+package diff
+
+import (
+	"fmt"
+	"strings"
+
+	coreauth "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/auth"
+)
+
+// BuildAuthChangeDetails computes a redacted, human-readable list of auth field changes.
+// Only prefix, proxy_url, and disabled fields are tracked; sensitive data is never printed.
+func BuildAuthChangeDetails(oldAuth, newAuth *coreauth.Auth) []string {
+	changes := make([]string, 0, 3)
+
+	// Handle nil cases by using empty Auth as default
+	if oldAuth == nil {
+		oldAuth = &coreauth.Auth{}
+	}
+	if newAuth == nil {
+		return changes
+	}
+
+	// Compare prefix
+	oldPrefix := strings.TrimSpace(oldAuth.Prefix)
+	newPrefix := strings.TrimSpace(newAuth.Prefix)
+	if oldPrefix != newPrefix {
+		changes = append(changes, fmt.Sprintf("prefix: %s -> %s", oldPrefix, newPrefix))
+	}
+
+	// Compare proxy_url (redacted)
+	oldProxy := strings.TrimSpace(oldAuth.ProxyURL)
+	newProxy := strings.TrimSpace(newAuth.ProxyURL)
+	if oldProxy != newProxy {
+		changes = append(changes, fmt.Sprintf("proxy_url: %s -> %s", formatProxyURL(oldProxy), formatProxyURL(newProxy)))
+	}
+
+	// Compare disabled
+	if oldAuth.Disabled != newAuth.Disabled {
+		changes = append(changes, fmt.Sprintf("disabled: %t -> %t", oldAuth.Disabled, newAuth.Disabled))
+	}
+
+	return changes
+}