Improve comments and Swagger annotations across services (#26)

- EmailController: add class summary, full SwaggerResponse/ProducesResponseType
  for 400 and 500, and Description on SwaggerOperation
- ContactController: fix terse "Failed." error message to
  "Could not process subscription."
- FileDownloadController: remove redundant XML <response code> tags from
  the public action doc block; convert private-method /// <summary> to //
  (project convention: no XML doc on internal code)
- CvMatcherService: remove two dead commented-out blocks (old email send
  and BuildEmailBody helper)
- JobTokenService: comment the phone/contact-line regex filter in
  ExtractKeywords
- DocumentClassifier: comment the keyword-frequency scoring approach and
  the confidence formula
- TextChunker: comment the sliding-window step (chunkSize - overlap)
- CvSearchJobTask: comment the GdprConsent = true rationale and the
  BuildCvFileName sanitisation logic
- HtmlJobSearcher: comment GetLeftPart(UriPartial.Path) query-strip dedup

Closes #26

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Jobs/cv-search-job/Tasks/CvSearchJobTask.cs

@@ -125,6 +125,7 @@ public sealed class CvSearchJobTask : IJobTask
                 {
                     CvDocumentId = session.CvDocumentId,
                     JobUrl = url,
+                    // User already gave GDPR consent when they clicked the one-time job search link
                     GdprConsent = true
                 };
 
@@ -191,6 +192,7 @@ public sealed class CvSearchJobTask : IJobTask
 
     private static string BuildCvFileName(string cvDocumentId)
     {
+        // Strip non-alphanumeric characters so the filename is safe for all OS/email clients.
         var safeId = string.Concat(cvDocumentId.Where(char.IsLetterOrDigit));
         if (string.IsNullOrWhiteSpace(safeId)) safeId = "cv";
         return $"{safeId}.pdf";