docs: correct comments and changelog wording

- FilenameSanitizer class doc previously attributed the bug to "shell
  globbing, URI handling, and various download clients"; that was
  speculation about the mechanism. The confirmed cause is a failure
  inside the Bezzad Downloader NuGet package on Linux. Rewrite the
  class doc to describe what we actually know.

- SanitizeFilenameIfEnabled / SanitizePathIfEnabled said "returns it
  unchanged" when disabled, which isn't accurate for null input: we
  coerce to String.Empty so the return stays non-null. Spell that out.

- SanitizeFilename method summary rewritten to list exactly what it
  does rather than "problematic characters".

- Changelog entry said the setting is "on by default" but the default
  is OperatingSystem.IsLinux(). Fix the wording.

CHANGELOG.md

@@ -6,7 +6,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased
 ### Added
-- Filename sanitizer that strips `[ ] { }` and control characters and collapses runs of whitespace so downloads no longer fail on Linux containers when the debrid provider returns filenames with square brackets (e.g. `[eztv]`). Gated by a new `Sanitize filenames` setting (on by default).
+- Filename sanitizer that strips `[ ] { }` and control characters and collapses runs of whitespace, working around a failure in the Bezzad Downloader NuGet package that blocks downloads on Linux when the debrid provider returns filenames with square brackets (e.g. `[eztv]`). Gated by a new `Sanitize filenames` setting; defaults to on for Linux hosts and off elsewhere.
 
 ## [2.0.129] - 2026-04-06
 ### Added

server/RdtClient.Service/Helpers/FilenameSanitizer.cs

@@ -5,10 +5,14 @@ using RdtClient.Service.Services;
 namespace RdtClient.Service.Helpers;
 
 /// <summary>
-/// Sanitizes filenames to prevent issues with special characters on Linux containers.
-/// Path.GetInvalidFileNameChars() on Linux only returns NUL and '/', so characters
-/// like [ ] { } that cause problems with shell globbing, URI handling, and various
-/// download clients pass through the built-in filter unchanged.
+/// Works around a known Linux-only failure in the Bezzad Downloader NuGet
+/// package, which throws "Access to the path is denied" when the target
+/// path contains square brackets, curly braces, or runs of consecutive
+/// whitespace — all common in debrid-provider release names.
+///
+/// Path.GetInvalidFileNameChars() on Linux only returns NUL and '/', so
+/// these characters pass through .NET's built-in filter unchanged.
+/// Control characters are also stripped as a defensive measure.
 /// </summary>
 public static partial class FilenameSanitizer
 {
@@ -23,7 +27,9 @@ public static partial class FilenameSanitizer
     public static Boolean IsEnabled => Settings.Get.DownloadClient.SanitizeFilenames;
 
     /// <summary>
-    /// Sanitizes a filename if enabled in settings; otherwise returns it unchanged.
+    /// When sanitization is enabled, returns <see cref="SanitizeFilename"/>.
+    /// When disabled, returns the input as-is; null is coerced to empty string
+    /// so callers can treat the return as non-null.
     /// </summary>
     public static String SanitizeFilenameIfEnabled(String? filename)
     {
@@ -31,7 +37,9 @@ public static partial class FilenameSanitizer
     }
 
     /// <summary>
-    /// Sanitizes a full path if enabled in settings; otherwise returns it unchanged.
+    /// When sanitization is enabled, returns <see cref="SanitizePath"/>.
+    /// When disabled, returns the input as-is; null is coerced to empty string
+    /// so callers can treat the return as non-null.
     /// </summary>
     public static String SanitizePathIfEnabled(String? filePath)
     {
@@ -39,8 +47,9 @@ public static partial class FilenameSanitizer
     }
 
     /// <summary>
-    /// Sanitizes a filename by stripping problematic characters and normalizing whitespace.
-    /// Does NOT touch directory separators — only a single filename segment.
+    /// Strips square brackets, curly braces, and control characters, collapses
+    /// runs of whitespace to a single space, and trims. Operates on a single
+    /// filename segment; does not interpret directory separators.
     /// </summary>
     public static String SanitizeFilename(String? filename)
     {