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/rag-api/Services/TextChunker.cs

@@ -10,6 +10,8 @@ public sealed class TextChunker : ITextChunker
         chunkSize = Math.Clamp(chunkSize, 300, 3000);
         overlap = Math.Clamp(overlap, 0, chunkSize / 2);
 
+        // Sliding window: step forward by (chunkSize - overlap) each iteration so
+        // adjacent chunks share `overlap` characters, preserving cross-boundary context.
         var chunks = new List<string>();
         var start = 0;
         while (start < text.Length)