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>

Apis/cv-matcher-api/Services/JobTokenService.cs

@@ -92,6 +92,7 @@ public sealed class JobTokenService : IJobTokenService
             .Split(['\n', '\r'], StringSplitOptions.RemoveEmptyEntries)
             .Select(l => l.Trim())
             .Where(l => l.Length > 5 && l.Length < 200)
+            // Skip lines that are purely digits, spaces, and phone/contact punctuation (phone numbers, emails, etc.)
             .Where(l => !Regex.IsMatch(l, @"^[\d\s\+\-\(\)\@\.]+$"))
             .Take(5)
             .ToList();